Information about Tropical Fish Parasites

Parasites have arisen by evolution from free-living animals. They often resemble their ancestors a good deal, but in the course of evolution, they have become better adapted for a parasitic life. Some have developed organs such as suckers for hanging on. Many lay tremendous numbers of eggs because the chances that anyone egg will infect a new host are incredibly small. Parasites have generally changed biochemically and immunologically so that they can survive inside of another organism and not be digested or killed. Some of them depend on their hosts for compounds which their ancestors could synthesize.

Some parasites are harmful to their hosts; others are not. A parasite that might be harmful if present in large numbers might not be harmful if just a few have parasitized the host. There are different types of hosts. A definitive host is the host harboring the sexual, adult stage of the parasite. An intermediate host is one harboring an asexual or larval stage of the parasite. A parasite vector is an arthropod or other invertebrate that transmits the parasite from one vertebrate host to another. An infection is parasitism by an internal parasite, while an infestation is parasitism by an external one. A facultative parasite is able to live either free or as a parasite, while an obligatory parasite must be parasitic. The term life-cycle refers to the development of a parasite through its various stages. A reservoir host is a host other than the ones normally involved in the life-cycle. Interestingly, we still don't know exactly how intestinal parasites manage to survive without being themselves digested along with the other contents of the intestinal tract. As you can imagine, the inside of the gastrointestinal tract is a pretty inhospitable place to live, as it is full of acids and enzymes designed to digest anything passing through it.

Parasites are typically divided into two groups: ectoparasites, which live on the outside of the host (including the gills, mouth, skin and fin surfaces), and endoparasites, which live in the tissues, blood and/or organs (including the gastrointestinal tract).

Some parasites are transmitted directly from fish to fish, such as some ectoparasitic protozoa and skin/gill flukes, however, they often involve a free-living phase found in the water or tank substrate. This is called a direct life-cycle. Other larger parasites often have complex life-cycles involving two or more hosts, including a fish. This is called an indirect life-cycle. It is important to know this information, as it will have an effect on treatment methods and whether or not you will have success or failure when treating parasites.

I strongly suggest that you find a veterinarian who is knowledgeable regarding aquatic medicine who is willing to work with you. Since vets educated in aquatic medicine aren't always easy to find, as an alternative, I suggest that you find a veterinarian who is interested, but perhaps a novice, and encourage him or her to work with you. Almost all of the major veterinary conferences offer courses in aquatic medicine, and there are many texts and websites that offer excellent information for veterinarians. Armed with a basic knowledge of infectious diseases, a good microscope and a pathologist who can read fish samples, you and your vet can work together to diagnose and treat fish problems. And don't forget that many veterinary labs offer free consultations to their veterinarians who use them, and this can be a great source of information, as well. Parasitologists are also valuable in helping to diagnose certain parasitic problems.

While it may be possible to guess at the cause of many fish infections, valuable time may be lost in trying different medications instead of attempting to diagnose the problem by utilizing stool samples, skin scrapings, Gram's staining of lesions, culturing lesions or biopsying bits of tissue for histopathology (examination of tissue under the microscope by a trained veterinarian after the sample has been processed by slicing it into very thin specimens and then stained with agents to identify structures). In some cases, it may be more helpful diagnostically to sacrifice an individual showing signs in order to accurately identify the problem, especially in cases of pet retailers, breeders and tanks with valuable fish.

When it comes to treating fish parasites, there are only a handful of medications commercially available, others can only be prescribed by a veterinarian.

Clinical signs of parasitic disease in fish can be vague. A sick fish may be seen swimming along, it may swim more slowly, it may swim on its side, the gills may move more rapidly, the fins may be clamped or be droopy, and it may be losing weight. The fish may have white spots on the skin or gills or it may rub against tank plants, ornaments or gravel. Some fish may have the appearance of a bloated abdomen.

It must be noted that most parasitic diseases occur as a result of poor water quality. Most parasitic organisms are opportunistic and may be present all the time in the tank or on the fish in low numbers, and only cause disease when the fish is stressed. The amount of fish present in a tank, the water temperature, pH, lighting, type of filtration system and water chemistries all influence the health of the fish.

Another common cause of a parasitic outbreak is inadequate quarantine of new fish, although many aquarists are loathe to quarantine new fish prior to introducing them into their permanent home. Many of the common and more serious diseases do not show up for two to three weeks following transport, and unfortunately, this can result in serious problems being introduced into the established aquarium. A quarantine tank can be a simple as a bare aquarium with a heater and sponge filter. Three to six weeks of quarantine should allow owners to identify any sick fish prior to introducing them into the community tank. During quarantine, weekly treatments with formalin (25 mg/L) are reasonable and likely to avert serious illness in many cases.

At the first sign of illness, use your water test kit to make sure that the values are within the limits for the types of fish you are keeping. The first thing to do is to perform a partial water change, as that is an important safety measure. If it is possible, move any ill fish into an isolation tank to more closely observe them and prevent the possible spread of any infectious organisms.

When it comes to treatments, there are several options. Drugs or chemicals placed in the water are commonly referred to as "bath" treatments. Drugs delivered orally are generally mixed in the food, and are meant to deliver systemic effects. Injections may be utilized in cases where small numbers of fish are involved. Bath treatments are the most variable, in that a specific concentration of the chemical is placed in the water for a specific length of time. As a rule, lower concentrations are used for longer periods of time, and vice versa.

Salt, plain old sodium chloride, is often overlooked as the number one best chemical to manage fish health. For most purposes, simple, un-iodized table salt is adequate, although there is nothing wrong with using aquarium salt specifically sold for that purpose, such as sea salt, solar salt and salt sold for consumption by livestock.

There are three common ways to use salt in freshwater tropical fish medicine. As an aid to osmoregulation, salt may be added in a concentration of 0.5-1.0% (or 5-10 parts per million-ppm). This concentration is safe for several hours and up to several days. At this concentration, control of protozoan parasites can be achieved. Higher concentrations of salt for shorter periods of time may be used to eliminate protozoa.

Now, let's talk about the common parasitic organisms found in freshwater fish, and how to best treat outbreaks.

By far, the most commonly encountered parasite is the organism responsible for ich. Ich is also called white spot disease, due to the gross appearance of fish infected with this organism. Ich is caused by a ciliated one-celled protozoan called Ichthyophthirius multifiliis. Often, there are small white spots found on the skin, fins and gills. Each spot is caused by a parasite that lies just beneath the fish's transparent skin (epithelium). The small spots measure up to 1 mm in diameter, but with severe infections, several parasites may coalesce to form irregular white patches. Heavily parasitized fish will have many white spots and will often scratch against rocks and gravel and they may show increased gill movements. Fish heavily parasitized with ich may develop secondary bacterial infections since the slime coat and epithelium of the fish are compromised, allowing bacteria in the water to invade the skin. The organisms feed on body fluids and cells, and a thin capsule forms over each parasite while it is under the skin.

Ich can infect virtually all freshwater fish, including both cold water and tropical species. The life-cycle contains both free-living and parasitic stages. Mature parasites, feeding on the host tissues, break through the skin and fall away from the fish, where they form cysts on the floor of the aquarium. This is the reproductive stage and each cyst can divide many times, eventually producing many hundreds of the infective stage, called swarmers. These exit the cysts and swim off to find a fish host. If they do not find a host within several days, they die. The time necessary to complete the life cycle depends on the water temperature of the aquarium; the higher the water temperature, the faster the life cycle. At 70 degrees F, it takes about three or four days to complete the life-cycle, and at 50 degrees F, it may take up to five weeks to complete the cycle. At low temperatures, the parasite may become dormant for considerable lengths of time.

There are a number of effective treatments available to treat ich. Since the organism is found beneath the outermost layer of skin, most treatments are ineffective in treating the parasite on the fish and are aimed at controlling the free-living stages.

Diagnosis and treatment should be instituted as soon as it is discovered, as prompt treatment is imperative in controlling this organism. Ich can be introduced with fish, live plants and live food, so proper quarantine can often prevent this.

Interestingly, plain old salt (sodium chloride), which is often overlooked in favor of other drugs and chemicals that are often thought to be better and more powerful is one safe and very effective method of treating ich.

Now that we have learned about the life cycles of the common fish parasites, let's take a look at some other common parasites and what they do to aquarium fish. In the last issue, we took a close look at ich, the disease caused by Ichthyophthirius multifiliis, which is responsible for causing white spot disease. Ich has both free-living and parasitic stages. There are many different treatments available, and specific treatment must be chosen based on the type of fish that are infected and the type of environment in which they are maintained.

When it comes to treating fish parasites, there are some medications commercially available that work well. Others can only be prescribed by a veterinarian. Drugs or chemicals placed in the water are commonly referred to as "bath" treatments. Drugs delivered orally are generally mixed in the food, and are meant to deliver systemic effects. Injections may be utilized for cases in which small numbers of fish are involved. Bath treatments are the most variable, in that a specific concentration of the chemical is placed in the water for a specific length of time. As a rule, lower concentrations are used for longer periods of time and vice versa.

Interestingly, salt (plain old sodium chloride), which is often overlooked in favor of other drugs and chemicals that are often thought to be better and more powerful, is one safe and very effective method of treating ich, as well as many other external parasites. Salt is the number one best chemical to manage fish health. For most purposes, uniodized table salt is adequate, although there is nothing wrong with using aquarium salt specifically sold for that purpose, such as sea salt, solar salt, salt sold for consumption by livestock and Kosher salt. As an aid to osmoregulation, salt may be added in a concentration of 2-3 ppt (or 0.02-0.03 parts per million: ppm). This concentration is safe for most fish, and at this concentration, protozoa cannot live with this level of salinity. This is approximately one tenth the strength of seawater.

As a treatment for ich and some other protozoa, the infested fish should be dipped in salt water at 30 ppt (or 3.0 ppm, which is the salinity of seawater), for 30 seconds up to several minutes, or until the fish rolls on its side. At that time, the fish should be placed back in its normal salinity water. To mix this solution, use tank water and dissolve in 30 teaspoons of salt per gallon, then place the fish into this water, monitoring it the entire time. This is then repeated once daily for three treatments, with a 30-70 % tank water change between treatments. This treatment is different from most of the others, in that it is used to treat the organisms in the skin of the fish, and not the free-living stages found in the fish tank. Lower doses may be used as a continuous bath in the fish tank. At 5-10 ppt (0.5-1.0 ppm), fish can survive for several hours to several days, and this will effectively kill the ich organisms. At 4-5 ppt (0.4-0.5 ppm), freshwater fish can live for up to four weeks, and this will also break the ich life cycle. A bath of 4-5 ppt will effectively kill anchorworms and lice. It must be noted that a salinity of 5 ppt or greater will usually kill live plants. One ppt= 1 tsp/gallon, three ppt=1 tbsp/gallon. One pound/100 gallon pond water.

Formaldehyde (formalin), 37% may be used as a medicated bath as a treatment for ich and some other protozoa, using a bath for 12-24 hours, dosed at 1.0 ml per 10 gallons of water. This is repeated for three treatments, every three days. Partial water changes (30-70%) should be performed between treatments. Formalin may be added to the entire fish tank; however, because formalin is so toxic, it is preferable to use it in a treatment tank. Formalin removes oxygen from the water, so it is imperative that the treated water is well aerated. Formalin is a carcinogen (capable of causing cancer), so anyone handling this chemical must be educated about its safe handling. Formalin supplied for use by veterinary clinics to preserve tissues for histopathology is not adequate as it is too diluted that it is unacceptable and it is not FDA approved.

Malachite green is another treatment for ich. Dosed at 0.1-0.15 ppm, malachite green may be used as a bath for 12-24 hour treatments. This is usually repeated daily for three days, with a 30-70% water change between treatments.

Another treatment for ich and some other protozoa is victoria green and acriflavin. These chemicals are available as a commercially available tablet that "fizzes" as it dissolves. This formulation is also designed to prevent secondary bacterial infections. One dose may be all that is necessary; however, it is recommended that 25% of the water be changed every 24 hours if additional doses are needed after the initial dose. The carbon should be removed from the filter during treatment.

Copper is another treatment for ich, and for some, it is considered the treatment of choice. Safe use of copper depends on the total alkalinity of the water. If the total alkalinity of the water to be treated is less than 100 mg/L, use of copper sulfate is not recommended. Copper is extremely toxic to fish, so it must be carefully dosed. It will also kill invertebrates and live plants.

Older treatments often recommended increasing the tank temperature during treatment, as well as removing the carbon from the filters. The reason for increasing the temperature was to speed up the life-cycle of the organisms, since, as we have learned, treatment is aimed at killing the free-living phase of the protozoa.

Other protozoal parasites that are found externally are Ichthyobodo/Costia, Trichodina and Chilodonella. Often monogenetic flukes (such as Gyrodactylus) are involved. The most obvious symptom of this is a gray-white film of excess mucus that develops over the body and is especially noticeable over the eyes or areas that contain dark pigment on the skin. Along the flanks, reddened areas may occur, and sometimes the gills become swollen. Fish may show rapid movements, flashing, and they may have respiratory problems. But as the infestation progresses, the fish become listless and lie on the bottom of the tank, occasionally scratching against rocks or tank ornaments. Usually, excessive mucus is caused by more than one organism.

Trichodina and the related protozoa usually cause problems in fish that are overcrowded, stressed or suffering from poor husbandry. Trichodina attaches temporarily by an adhesive disc to the skin. They are considered commensal in low numbers. In large numbers, they damage the skin, become parasitic, feeding on the skin. Ichthyobodo attaches to the gills or fins, and feed on cytoplasm, causing hyperplasia of the epidermis and destruction of goblet cells.

Initially, these should be treated with the same therapy as for ich, as these are protozoal parasites (except for the flukes). If the fish have not improved within 5-7 days, perform a 50% water change and treat with formalin or an organophosphate insecticide used for removing the monogenetic flukes. These and other parasites will be discussed in more detail shortly.

Hexamita and Spironucleus are two other protozoal parasites commonly found in the intestinal tract of some freshwater fish, however systemic infections can occur. Hexamita is also known as "hole in the head disease." It may cause small holes to appear in the body, especially the head region, and often strings of mucus trails from the lesions. Lesions may also develop at the base of the fins and near the lateral line. They have a direct life cycle, and are principally acquired by oral ingestion of trophozoites or cyst stages.

In addition to the skin lesions fish often go off their food and develop a hollow-bellied appearance, and often have stringy, off-color feces. Hexamita often exists in low levels in the intestines of tropical fish, especially cichlids, including discus, angelfish and oscars, and gouramis. They may be considered commensals, however, with stress, overcrowding, low oxygen levels, improper water parameters, changes in temperature or poor diet, the parasites can multiply and the fish may then develop signs of systemic disease.

The best way to treat Hexamita infections is with medicated food. However, since infected fish often become anorexic, they usually won't ingest an effective dose. Both Hexamita and Spironucleus are best treated with metronidazole. This medication is available in commercially available fish food flakes. If the infected fish are still eating, they should be treated for 5-10 days, with 10-15 mg/gram of food. If the fish are not eating, baths should be used, immersing the fish for 6-12 hours, with 250 mg/10 gallons of water. Several courses of treatment may be necessary to successfully control this problem. Of course, as with all infectious organisms, it is always best to prevent introduction to the tank in the first place, by quarantining and prophylactically treating all new stock, and to maintain your fish in the most healthful manner possible.

A parasitic single-celled organism known as a dinoflagellate, Piscinoodinium (also commonly called Oodinium) is referred to as velvet disease. This parasite has a direct life cycle and has free-living and parasitic stages. The obvious symptoms are a yellow-gray coating to the scales, skin and fins. Fish may flash, rubbing against rocks, and show increased gill movements. In advanced cases, the fish will become anorexic and float motionless in the water, and the skin may peel away in strips. This disease may be confused with ich by the novice, but the fish with velvet appears as if it is sprinkled with gold dust; hence the common name of "gold dust disease." This parasite moves from fish to fish with a flagellated spore, and it can live off of the fish for at least 24 hours, and probably longer, up to several days. It is usually introduced with new fish, and may then develop into a serious problem (again, emphasizing the importance of quarantine and prophylactic treatment during that time). Heavy infestation on the gills may kill fish without causing other obvious signs of the disease. Velvet is most often found among some anabantids, killifish and goldfish. The parasite may establish itself in the intestinal tract of fish, making treatment more difficult, and long-term control of the parasite can be quite frustrating.

Treatment with antiprotozoals medications, treatment for ich or velvet remedies may be used. If the fish are salt-tolerant, a prolonged salt treatment, using one teaspoon per five gallons, can be used to eradicate the parasite. Interestingly, since the parasites utilize photosynthesis, by keeping the aquarium relatively dark, this can help during a serious outbreak. Darkness will prevent the parasites from photosynthesizing and will also delay the development of dinospores.

A microsporidian protozoal parasite, Pleistophora, is responsible for neon tetra disease. As with many protozoal problems, fish may carry low levels of this organism without overt signs of disease. Heavily infected fish will show a loss of coloration (especially the red stripe on neons), unusual swimming behavior, spinal curvature, emaciation and frayed fins. A range of fish species are susceptible, however, this parasite seems to affect many tetras preferentially. The zebra danio and some barbs are also commonly affected by a similar disease.

Treatment has been attempted with a number of antiprotozoals; however none has been completely effective. Some medications will help control secondary bacterial infections. New drugs are continually being developed and it is a good idea to check current proceedings from veterinary conferences for new medications on the horizon.

Several microsporidian and myxosporidian parasites, including Ichthyosporidium, Nosema, Myxobolus, Dermocystidium and Henneguya can cause nodular skin diseases or internal nodules in tropical fish. Yellowish or white cysts, often called xenomas on the skin, gills, fins, in muscles or in internal organs may range in size from a few millimeters to a centimeter in diameter. Because of their small size, electron microscopy is usually needed to identify the parasite genus. The life cycles of these parasites are poorly understood, but they are thought to be direct. It is thought that resistant spores help spread these protozoa. These parasites can be species specific, so cross infections do not often occur. Internal nodules may not be noticeable; however, lesions on the skin, gills and fins are visible. There is no reliable treatment for these types of protozoa.

Monogenetic flukes are flatworms that are usually found as ectoparasites of fish. They often have a hooked attachment organ and they have a simple, direct life cycle. They are just visible to the naked eye. They often feed on skin and gill tissue, but usually only cause problems if found in large numbers. Gyrodactylus lives on the skin, fins and gills of many species of fish. This fluke gives birth to live young (others lay eggs). Clinical signs are "flashing" and skin problems. Treatment with formalin, praziquantel, TrichlorofonTM (dimethyl phosphate) or salt baths are effective. Formaldehyde is used as a bath for 12-24 hours at 20-25 ppm (mg/L), repeated every three days, for three treatments (same as for ich), with 30-70% water change in between treatments. Praziquantel is dosed as a bath for 3-6 hours, at 5-10 mg/L, repeated three times, with 30-70% water change between treatments. Praziquantel is now available in several commercially prepared tablets. TrichlorofonTM is used as a bath, for one hour, 0.25-1.0 ppm, repeated daily for three days, with 30-70% water change between treatments. Salt is used as a bath, as a 4-5 minute dip dosed at 30-35 g/L, repeated daily for three treatments, with a 30-70 % water change in between treatments.

Digeneans are endoparasitic flukes. They have two suckers, one at each end. They have an indirect life cycle, with fish acting as both intermediate hosts (carrying metacercaria larvae) and final hosts. Adult digeneans are usually found in the gastrointestinal tract, the larvae (which may be encysted) can be found throughout the body and cause problems if they invade such organs as the eyes (as does Diplostomum) or heart, in high numbers. The larval stages of digenetic fluke parasites (Clinostomum, Posthodiplostomum and Diplostomum) cause black spot in aquarium fish. Praziquantel is the treatment of choice. As a bath, for 3-6 hours, it is dosed at 5-10 mg/L, repeated for three doses, with a 30-70% water change between dosing. Praziquantel may also be dosed in the feed, at 5 mg/kg of fish, or it may be injected intraperitoneally or intramuscularly, dosed at 5 mg/kg.

Tapeworms (cestodes) are endoparasites. Adult tapeworms live in the digestive tract. Tapeworms are most common in newly imported or wild-caught fish. They have an indirect life cycle, with fish serving as both intermediate and final hosts. There is a public health concern, as some cestodes can be infective to humans, causing zoonotic infections. Diphyllobothrium has a larval stage that can be fatal to fish and infective to humans. Signs of tapeworms may be a swollen abdomen, general unthriftiness, and wasting. Fish may have difficulty swimming. Treatment involves using praziquantel, which will remove tapeworms from the intestinal tract, but will not eliminate those residing within the body cavity. Treatment with praziquantel is the same as for digeneans. Avoid feeding live foods such as cyclops and water fleas, which are the intermediate hosts of many of these parasites, unless you are certain that these come from a fish-free water source. Repeating treatment with praziquantel may be necessary to completely eradicate tapeworms.

Roundworms may also be found in aquarium fish. Also called nematodes, they have complex indirect life cycles, and fish can serve as both intermediate and final hosts. The developmental cycle of nematodes comprises egg, four larval stages and adults. Nematodes can cause problems to fish, and also to humans who become infected by eating raw or poorly prepared fish. Roundworms are red-brown in color and vary in size. In severe infections, they may be seen protruding from the vent. Nematodes can be found in the digestive system, swimbladder and body cavity. Treatment involves breaking the life cycle and using appropriate anthelmintic drugs, such as fenbendazole. Fenbendazole is dosed in feed for three days, at 200 mg/100 gm of food. It may require several rounds of treatment, in addition to eliminating any intermediate hosts and improving general hygiene to remove infective stages. Piperazine can be used to treat livebearers with Camallanus nematodes. Mix 25 mg of piperazine citrate with 10 gm of flake food, and feeding this to the fish for 5-10 days. A repeat course of treatment is recommended 10-14 days later (dosed at 50-100 mg/kg of body weight).

Some companies are now selling tablets with a combination of medications, designed to eliminate many of the common internal and external parasites, including praziquantel, metronidazole and a medication to prevent secondary bacterial infections.

Acanthocephala, also called spiny or thorny headed worms, are highly specialized for life in the digestive tract of the host. They have spines on the evertible proboscis at the anterior end of the body, by which the worm anchors to the tissues of the digestive tract. The life cycle is indirect, with crustaceans serving as the intermediate host. They can cause damage by puncturing the intestinal tract or blocking the passage of food through the gastrointestinal tract. Acanthocephalans rarely cause problems with captive fish, although wild-caught fish may harbor them when first obtained. Because of the complex life cycle involving intermediate hosts, re-infestation is not likely to occur in the fish tank. After some months, these parasites are likely to die off and disappear. As long as the worms do not perforate the intestines, the damage is self-limiting.

Some crustaceans are parasitic to fish. The fish louse, Argulus, and the gill maggot, Ergasilus, are both crustaceans that parasitize fish. The fish louse attaches itself to the skin and fins by means of twin suckers, and it feeds on blood of the host. The intense irritation may cause the fish to scratch against rocks and heavily infested fish may even jump out of the water! Red lesions occur where the lice have attached, and this opens the skin up to secondary bacterial and fungal infections.

Gill maggots are usually found attached to the gills, gill covers and inside the mouth. Heavy infestation can result in severe gill damage, emaciation, anemia and death. Both of these parasites are usually found on newly imported fish. Because they suck blood, they can transmit certain infections between fish.

Treatment requires an organophosphate insecticide, and the drug can be applied to the pond or aquarium to eradicate both adult and juvenile crustacean parasites. Baths of TrichlorfonTM and dimilin are both effective treatments. Dimilin may be used as a bath for 48 hours, at 0.01 mg/L and repeated up to three times, and TrichlorfonTM can be used as a bath for one hour, at 0.25-1.0 ppm, repeated up to three times.

Anchor worms are another crustacean parasite, Lernaea. Anchor worms are elongated parasites with two egg sacs at the posterior end. They usually embed in the muscle of the body wall and often penetrate as far as the internal organs. A raised ulcer usually develops at the point of attachment, and secondary infections often occur at that site. Heavy infestations may cause weight loss and death.

Anchor worms are most often a problem with newly imported fish and in garden ponds in the summer, and don't often occur in aquariums. Males die after mating and have a short lifespan, and females are usually seen attached to the fish host. The eggs hatch to produce free-living juvenile parasites, which eventually molt to produce the adult stages. The juvenile stages can live without a host for at least five days. As with other crustacean parasites, organophosphate insecticides are used to eliminate the free-living juvenile stages. Adult worms should be manually removed with forceps, and the site treated with a suitable antiseptic. Parasites can be removed weekly until no more adult worms are found.

Blood parasites, including Trypanosoma and Trypanoplasma require a leech vector to transmit them from fish to fish, so these rarely cause problems in aquarium fish. The best way to deal with these is to eliminate the leech vector. Fish with blood parasites may develop anemia, listlessness, emaciation and pop-eye. Heavy infestation may result in death.

Obviously, it is impossible to discuss every type of parasite that can attack fresh water fish, but we have covered the most commonly encountered parasites. While you may be able to diagnose and treat the more common parasites, it can really be beneficial to you to establish a good working relationship with a veterinarian who is willing to help you in diagnosing and treating your fish problems. But, please remember that prevention is always better than trying to treat and cure problems, and quarantining is your best way to protect your established fish.


PPM: parts per million
To calculate the amount of salt needed (in grams)= volume (gallons) x 3.8 x ppt desired
To convert this to the number of pounds of salt needed divide the final number by 454 (454 grams= 1 pound)
1 PPT= 1 tsp. of salt per gallon
3 PPT=1 tbsp. of salt per gallon


External Parasite


Ichthyophthirius multifiliis & Trichodina & Ichthyobodo &Tetrahymena

Salt, formalin, malachite green, other commercial products, inc. temp.

Gill flukes, skin flukes

Copper, malachite green, formalin, others

External protozoa, flukes

Malachite green and formalin, others

Velvet (dinoflagellates)

Antiparasite medication, salt, copper, inc. temp

Fish lice, gill maggots

formalin, potassium permanganate, metriphonate, other commercial prod.


organophosphate, manual removal and then dab with antiseptic

Larval stage of digenetic fluke parasites

organophosphate, other commercial products

Internal Parasite


Hexamita & Spironucleus

Metronidazole, other commercial products

Blood parasites

If flukes: praziquantel, others difficult to treat

Nematodes (roundworms)

Use of appropriate anthelmintic drug, eliminate IM host, remove dead fish, piperazine

Spiny-headed worm

Appropriate anthelmintic



Digenetic flukes

Difficult to treat, praziquan

Article courtesy of