Looking after Your Oscar's Health

A lot of fish keepers seem to be under the misapprehension that keeping tropical fish is easy and it's just a case of filling the tank up with water, chucking a few fish in and then feeding them two or three times a day. If you want to keep your fish alive and healthy then you as a fish keeper have got to do your bit. It doesn't matter whether the tank is 5 gallons or 500 gallons, you will still need to put in quite a lot of effort to keep the tank clean and healthy.

Fish illness checklist

If for any reason you think that one of your fish maybe ill then please use this checklist to identify any symptoms that your fish may be showing.

The number one cause of fish death is probably toxic poisoning by ammonia. In many cases, this happens in a tank that is in the process of cycling. However, it doesn't take a lot to upset the biological filter which in turn will allow lethal amounts of ammonia to build up in the water. It may not be obvious straight away that this is happening, however, before long you will start noticing signs that all is not right. Below is some information which will help you recognise signs of fish being poisoned by ammonia and also nitrate.

Ammonia poisoning and stress

Ammonia stress and ammonia poisoning are basically the same. However, the term ammonia poisoning is normally used to describe instances where the fish have actually died. When the fish are showing signs of being poisoned by ammonia we use the term ammonia stress.

What are the signs of ammonia stress?

Until levels of ammonia get quite high the fish may not show any signs at all. The pH of your water may also determine how toxic the ammonia is. The pH level below 7.0 will greatly reduce the toxicity of ammonia. Alternatively, an alkaline pH can make ammonia more toxic. This is why having a low pH is always beneficial.

How to reduce ammonia stress and poisoning

It's quite simple, carry out a water change in order to reduce the amount of ammonia in the water. We wouldn't recommend relying on ammonia removers to control ammonia, you're much better off removing the ammonia manually altogether.

If the tank is in the process of cycling then carry out a water change every two or three days in order to bring the ammonia levels down to a safe level for the fish.

In a fully cycled tank, there should be no ammonia present. However, if you keep getting ammonia problems then there could be another problem. If you have too many fish for your size of tank then this is often the reason my people cannot completely get rid of ammonia in the water. The same goes for feeding your fish. If you put too much food in and a lot of it goes uneaten then you may find it breaking down and causing ammonia spikes.

Nitrite poisoning

Nitrite poisoning follows very much in the steps of ammonia poisoning since nitrite is a byproduct of ammonia and is nearly as toxic. If you have problems with ammonia then there is a very good chance that you will also follow up with and nitrite spike. The symptoms of nitrite poisoning are basically the same as ammonia. You should follow the same procedure as ammonia poisoning to bring nitrite levels down. Nitrite spikes can often occur if somebody introduces too many fish at once, even in an established aquarium. Feed very sparingly until nitrite levels returned to 0 ppm.

Nitrate poisoning

Nitrate poisoning occurs when fish are exposed to rising nitrates in their aquarium. The majority of fish can actually withstand nitrate levels up to 100 ppm. However, once they exceed these levels the fish may start showing signs of stress and illness.

Nitrate shock occurs when fish are exposed to huge changes in nitrate levels in a very short period of time. A good example would be when a fish keeper buys a new fish from the fish store where nitrate levels are kept low. They then take the fish home and put it in their own aquarium where nitrate levels are very high. Nitrate shock can kill fish in a matter of 24 hours. Nitrate shock can also occur in a mature aquarium. If nitrate levels are already well in excess of 100 ppm and a very large water change is carried out, the sudden drop in nitrate levels can have a devastating affect on your fish stock. So it's important to bring down high levels of nitrate in segments over a few hours in one day, rather than all at the same time.

Here are a few symptoms of high nitrates to watch out for .

  • Loss of appetite
  • Fish may become listless
  • Fish may lose its balance in the water
  • Fish may lay on the bottom of the tank
  • Fish may exhibit heavy breathing

Keeping nitrate levels low is quite easily if you follow these simple rules

  • Regular water changes
  • Don't overfeed
  • Don't overstock your aquarium

How to euthanise your Oscar Fish

It's not always possible to cure an Oscar if it's been suffering from an illness. Occasionally, it may be kinder to euthanise the fish. We have a very good article on how to euthanise a fish using clove oil. I got the procedure directly from a veterinary surgeon so it's a tried and tested method that will ensure you can euthanise your fish without causing it any undue suffering.

Listed here are some common symptoms of the three most likely causes of fish illness.

Bacterial Infections: lethargic (often inactive for long periods of time), bloated body, cloudy eyes, bulging eyes, abscesses, red streaks throughout its body, loss of colour, open sores, inflammation or reddening of the skin.

Fungal Infections: fish will swim erratically, scratching, visible cotton like substance on the skin, around the eyes or mouth. The fish may also dart back and forth for no reason.

Parasitic Infections: lethargic, excess mucus, or slime on the body, visible worms or white spots, scratching against rocks or wood, breathing heavily.


Grayish-white film on skin, damaged fins, ulcers, yellow to gray patches on gills, tissue on head may be eaten away. Columnaris (Cotton Wool Disease) Must be treated immediately with Over-the-counter antibiotic medications. Very contagious disinfect tank, rocks, net, etc.
Swelling of head, bulging eyes. Corneybacteriosis OTC antibiotics such as penicillin and tetracycline.
Swelling of abdomen, raised scales around swollen area. Dropsy (Malawi Bloat) may be caused by internal bacterial infection (if swelling is sudden), parasites, or cancer (if swelling is gradual). Add 1/8 teaspoon of Epsom salt for every 5 gallons of water and monitor for two weeks. Check for signs of bacterial infection or parasites for further treatment.
Ragged or decaying fins. Finrot Check pH and correct as needed. If level is normal, use OTC antibiotic for fin or tail rot.
Inactivity, loss of color or appetite, weight loss, skin defects. Fish Tuberculosis Human strength TB medication may help in early stages. Contagious disinfect tank, rocks, net, etc. to prevent transmission. Wash hands and surfaces well.
Erratic swimming, bloating or swelling in body, black patches on body or fins. Myxobacteriosis -- rare Medications, if any, are difficult to come by. Keep up on water maintenance to prevent it.
Sluggishness, lack of appetite, fin damage, reddish discoloration, bulging eyes, clamped fins Septicemia Antibiotic treatment in food form is required.
White or gray fungus on eyes. Cataracts OTC medication for fungus.
White or gray patches resembling cotton, excess mucus. Mouth or Body Fungus OTC medication for fungus. Usually added to water, but may need direct application.
White cotton-like patches on fins, body, or mouth. True Fungus (Saprolegnia) OTC medication for fungus. Check for symptoms of other illnesses.
Small string-like worms visible on fish, or burrowed in skin. Anchor Worm Over-the-counter medication for parasites.
Bluish-white film on body, strained breathing caused by gill damage, peeling skin. Chilodonella Salt treatment (see below).
Weight loss, strained breathing. Copepods OTC medication for parasites, also fungal treatment for possible secondary infection on damaged gills
White film, reddened areas on body, abnormal swimming, scratching, folded fins. Costia (Slime Disease) Must be treated quickly. Raise water temperature and use OTC medication for parasites. Salt treatment may work, as well.
Weight loss, abnormal swimming, generally looks very ill. Will accompany or follow leech infestation. Blood Flagellates (Sleeping Sickness) rare Salt treatment can be used to kill leeches, but may not cure flagellates.
Sluggishness, flashing, spider web lesions on skin, color loss, reddened fins, drooping fins, fin damage. Skin Flukes (Gyrodactylus) OTC medication for parasites
Lack of appetite, weight loss, small holes or eroding pits appearing in the head. Hole in Head Disease (Hexamita) more common in cichlids Treat HITH ....
Scratching, white salt-like spots starting on head and spreading over whole body, rapid breathing, cloudiness on eyes or fins. Ich (Ichtyophthirius) very common OTC medication for Ich or Ick.
Scratching, small worms hanging from body. Leech Salt treatment or OTC medication for parasites.
Scratching, green to brown lice (up to inch) visible on skin. Lice OTC treatment for parasites.
Erratic swimming, weight loss, loss of color. Neon Tetra Disease mostly affects tetras, danios, and barbs Treatment is difficult look for a medication that treats gram-negative bacteria or with nalidixic acid as the active ingredient.
Darting, scratching, small yellow to white spots dusting skin. Oodinium OTC treatment for parasites.
Cloudy appearance on skin, red patches on skin where parasite has bitten. Trichodina -- predominately freshwater Salt treatment.
Red or bloody gills, gasping for air. Ammonia Poisoning No treatment. Regular water testing and maintenance will prevent it.
Small dark spots on fins and body. Black Spot OTC medication for parasites. Spots (cysts) may remain after treatment.
Cloudy white appearance to one or both eyes. Cloudy Eye Check for symptoms of another illness like velvet, ich, or tuberculosis. Treat with OTC medication.
String of feces hanging from fish, swollen abdomen, sluggishness, disinterest in food, off-balance swimming. Constipation Stop feeding for 2-3 days and continue with a more varied diet including live and plant-based foods.
Small white spots that get larger over time possibly with black streaks. Fish Pox No treatment. Keep up on water maintenance and symptoms should cease after about 10-12 weeks.
Difficulty swimming, swimming upside-down, floating, unable to surface. Do not confuse with swim bladder disease. Flipover Air can be removed from swim bladder by a veterinarian. Surgery is also a possibility in larger fish. Check for signs of internal infection or parasites and treat as necessary.
Reddening on or under skin, sudden abnormal behavior. Inflammation OTC antibiotic treatment.
Unusally bulging of one or both eyes. Pop-eye (Exophthalmia) OTC medication for bacterial infections and/or parasites. Check for other symptoms of bacterial or parasitic infections.
Fish struggles to swim, may float with head tipped down, or have difficulty surfacing, no balance, etc. May occur after eating. Swim Bladder Disease Stop feeding for 3-4 days. If symptoms persist, feed the affected fish a small amount of fresh spinach or a green pea without the skin (laxatives).
Swelling or distention for internal tumors, external can be seen growing on skin. Tumors Usually incurable. Consult a veterinarian about potassium iodide treatment for thyroid tumors.
Sluggishness, lack of appetite, open sores with red edges, possible fin rot. Ulcers OTC medication for bacterial infections.
Scratching, small gold to white spots, loss of color, weight loss, difficulty breathing due to gill damage. Velvet (Gold Dust Disease) OTC medication for parasites.

Symptoms chart courtesy of fishnet.org