Setting up an Oscar Fish Tank

Before rushing out to buy your first Oscar take some time to look at what is involved in keeping one of these fish. Surprisingly, Oscars can live for over 10 years, that's the same as a pet dog. So it's important that you understand that getting an Oscar is a long-term commitment to looking after your fish's welfare. Your Oscar is going to rely solely on you to make sure that it's home is clean and safe at all times. Your commitment to your Oscar will mean carrying out tank maintenance at least once a week for the entirety of your fish's life. Changing water and cleaning substrate shouldn't take more than one hour every week, but it will be a task that cannot be overlooked or ignored.

Aquariums cost money to run

You should consider the cost of keeping a large aquarium running 24/7. You will need a heater and a filter running every single day of the year. Because Oscars are tropical fish the aquarium water must be kept warm at all times. A 55-gallon aquarium is going to require a minimum of 200 W of heating that must be switched on all the time (imagine leaving the house lights on 24/7 in two or three rooms of your house). Filters won't use as much power, but they will still need to be kept running at all times. Many of us have water meters and therefore pay for what you use. The bigger the tank, the bigger the water change each week. Then you've got your food on top of all this, plus any medication that may be needed. The biggest mistake you can make is thinking that an aquarium doesn't cost anything to run. Big aquariums can not only increase your electric bill each year but also the water bill at the same time.

The correct size aquarium

Having the correct size aquarium for an Oscar is absolutely paramount. You wouldn't buy a Saint Bernard dog if you lived in a tiny one-room flat, would you? So why people think they can house a large Oscar in a 25-gallon tank is anyone's guess. Remember that Oscar fish can reach 12 inches + as adults. That's a large fish that needs a fairly big aquarium. One adult Oscar must have at least 55 gallons of water to itself. If you are able to get a larger aquarium then we would always advise you to do this. Keeping tropical fish is highly addictive and once you've got one fish, you normally want more. So what if you want two Oscars or several tankmates? Start thinking about an aquarium in excess of 100 gallons. I can absolutely promise you that trying to keep two Oscars in anything less than 100 gallons will result in extra tank maintenance. You'll find yourself having to change the water more than once a week and carry out filter cleaning more often. The worst thing you can do is give yourself too much work to do because after a while you will get bored with it and the chances are the fish will then start getting neglected. Aquariums come in many shapes and sizes. When choosing a tank for your Oscar you should opt for a tank that will allow the Oscar to swim back-and-forth quite easily. Therefore, avoid dome and hexagon -shaped aquariums, they don't have enough length in them to allow the Oscar room to swim freely.

If you are buying from the fish store then they will be able to tell you exactly how many gallons the tank holds. However, if you are buying second-hand then it's not always apparent how many gallons the tank holds. Using this simple aquarium size calculator, you can enter the dimensions of the tank and it will give you an accurate reading of exactly how many gallons/litres the aquarium holds.

There are a few companies who manufacture very large aquariums. Very large aquariums are by no means cheap, but if you are intending on keeping lots of big fish then you're going to need one of these very big tanks. Check out fish tanks, they manufacture aquariums in excess of 350 gallons. You can customise the aquarium and have all your filtration system fitted as well. If you are going to get serious about keeping Oscars and large fish then this is probably the best direction to go.

US/UK gallons converter

If for some reason you want to convert US gallons into UK imperial gallons, or vice versa than this UK/US Gallons Converter chart is very handy

Always use a secure aquarium hood

I really can't emphasise enough how important it is that you have a secure lid on your aquarium if you are keeping Oscar fish. For some reason Oscars like jumping. I believe that in the wild they will often jump out of the water to catch insects from overhanging leaves and branches. They seem to replicate this in the aquarium as I've seen them jumping out of the water to catch small flies. Even a medium-size Oscar can lift the lid a couple of inches on a 125-gallon aquarium. Some tanks don't have lids, they just have the lights suspended above the water. These are most definitely the worst design aquarium you could ever use with an Oscar. If you want to avoid finding your Oscar dried up on the floor in the morning then always use a secure lid on your fish tank.

The importance of good filtration

The filter is the absolute heart of your aquarium. The filter is very much like the liver or the kidneys, it filters out all the horrible stuff and makes sure that your water is always nice and clean in order for your fish to stay healthy. It's very important that the filter you use is able to circulate the water quick enough in order to remove all those dangerous toxins that can easily kill fish. For instance, if your tank is 60 gallons, make sure that you are using a filter that is rated for more than 100 gallons. I think it's fair to say that we tend to go a little bit overboard when keeping Oscars. Oscars are messy, they produce a lot of waste so you need that extra filtration.

Knowing how your filter media works

I think it's important that you understand what's inside your filter and how the different elements work to keep your water clean. All filters come with mechanical and biological filtration, some also contain chemical filtration. All three types of filter media have their own job to do and knowing what each one does is very important.

Mechanical filtration

Mechanical media normally comes in the form of a piece of sponge, often blue in colour. This type of media is there to catch any solid pieces of debris that the filters suck up. The debris can be anything from fish poo through to the uneaten fish food. Depending on what size tank you've got, and how many fish you are keeping will determine how often you will need to remove the mechanical filtration for cleaning. Cleaning mechanical filtration is very straightforward, you can take it out of the filter and give it a rinse out in a bucket of existing tank water. This type of mechanical filtration can be changed every few months when it gets absolutely filthy.

Biological filtration

Biological media is what really keeps the whole aquarium alive. It's what removes all the toxins from the water and keeps the fish alive. Without biological filtration, your fish would be swimming around in a toxic soup. If you are using a canister filter then you will probably find it came with some strange -looking bags of material that either resembles little round balls, or little rings that look a bit like pasta. This, in fact, is your biological media and will be contained within the trays in the filter. This type of filter media is very porous and therefore the nitrifying bacteria have a really good surface to live on. The big advantage of using this type of biological media within a canister filter is that it really does an excellent job of dealing with ammonia and nitrite. Secondly, if you don't overstock your aquarium then you can often leave canister filters for longer intervals in between cleaning.

Chemical filtration (activated carbon)

You may have noticed a black sponge in your filter. This is what is called chemical filtration and it's designed to remove impurities from the water. The best use you can get from using chemical filtration is to remove any medication that is left over from a water treatment. Some people use it in order to stop their water smelling. However, the problem with chemical filtration is that eventually, it will become completely saturated with impurities. If this happens then there is a possibility that impurities could start leaching back into the aquarium water. It's up to you whether you use chemical filtration. If you do use it then be aware that it needs replacing every few months.

Which filter to use

There are only two types of filter I would recommend for an Oscar fish aquarium, and they are a large canister filter or a sump filter. Because Oscar fish require large aquariums, small filters are of little use because you would need about three or four of them installed in the aquarium. Even then, you probably wouldn't have adequate filtration. There are numerous different manufacturers of canister filters that would do a great job in your aquarium. If you've got the money then I would recommend Eheim as they produce the Rolls-Royce of aquarium filters. Yes, they are expensive, however, you get exactly what you pay for. Eheim produces canister filters that will filtrate aquariums right up to 1200 L, which is about 300 gallons. I've used Eheim filters ever since I got my first aquarium and I think they are just fantastic. Another make of filter that a lot of people highly recommend is Fluval. They also produce good quality canister filters that are more than adequate for filtrate in a large aquarium containing Oscar fish. Because Oscars are so incredibly messy it's a good idea to literally double the filtration on your aquarium. If you've only got the one adult Oscar in a 55-gallon aquarium then I don't think there's any need to go over the top, a filter rated for a 100-gallon aquarium would work just great. When I had my 125-gallon aquarium I have a couple of Eheim 2028 canister filters installed, both of which are rated up to 132 gallons. But I had two Oscars and a few tankmates and the extra filtration was well needed. The beauty of having two canisters installed on the one aquarium is if one breaks down, you've still got a working filter that will still keep the tank clean until you get the other one fixed. Also, having to filters means that you can alternate filtration cleaning. This means that you are not cleaning both filters at the same time risking losing bacteria. You can clean one filter one week, and then wait another week and do the other one.

Keeping oxygen levels up

Canister filters normally come with a spray bar that resembles a garden sprinkler. The spray bar is attached to the side of the tank and should always be placed just above the surface of the water. What you are aiming to achieve is a constant water surface movement created by the water spray. It's important that your fish have plenty of oxygen, setting up your spray bars properly will create this surface agitation and in turn creates lots of little oxygen bubbles. When a fish tank hasn't got enough oxygen in the water the fish will normally breathe fairly heavily. If the oxygen depletion is particularly severe then the fish may hang at the surface opening and close their mouths whilst they take in oxygen. If you're using a good filtration system then it should oxygenate the water properly. However, if you feel that you want to add oxygen to the water, maybe because the weather is warm then you can use an air stone.

The sump filter

For fish keepers who have got especially large aquariums, the sump filter is often a good alternative to a canister filter. As I've already mentioned Eheim do produce a canister filter which is capable of filtering a 1200 L aquarium which is approximately 300 gallons. However, once you start getting even bigger, four or 500 gallons then a canister filter is just not going to be efficient enough. The beauty with sump filters is they are relatively easy to make and you can make them as big as you like. We have a page dedicated to the workings of the sump filter so have a read. Further Reading on Sump Filters...

Heating your aquarium

Oscar fish come from the tropics and therefore need to live in a heated aquarium. Oscars are okay with a water temperature as low as 21°C, and they can cope with temperatures of up to 30°C (temporarily I should add). A good temperature for Oscars all year round is 27°C. Oscars are known for being fairly rough with things that are placed inside their aquarium. For some strange reason, they often take a bit of a dislike to heaters and can easily smash a glass heater to pieces. Over the years I've had to replace the heater several times because of destructive Oscar fish. There are loads of heaters on the market that are made of heavy duty glass or even metal. These are the type of heaters that I would recommend for an Oscar fish aquarium. One of the best heaters I ever used was manufactured by a company called Aqua Medic. They manufacture single heaters up to 500 W which is perfect for aquariums up to around 125 gallons. They are made of titanium and are virtually indestructible, the Oscar would have to be on steroids to even make a dent in one of these heating elements. However, you also have to purchase the control box that goes with these heaters so they will end up being quite expensive. If you would prefer something a little more affordable than you could look at a new range of heaters called SCHEGO. They are also made of titanium and are extremely robust. Like the Aqua Medic, you also need to purchase a temperature controller because they don't have a built-in thermostat. However, the thermostat that controls the SCHEGO range of heaters is a lot cheaper than the Aqua Medic model. Your heater should always be submerged under water. I always like to place mine near to the water flow entering the aquarium. This will ensure that the whole aquarium is heated properly. One important thing to bear in mind is using the correct size wattage for your size of the tank. What you don't really want happening is the heater working overtime in order to keep the water warm, this will just cost you an absolute fortune and will probably lessen the life of the heater.

Aquarium thermometers

Aquarium thermometers were always the bane of my life, I never found an internal thermometer that the Oscars wouldn't find and destroy. The best advice I can give you is to completely forget about putting thermometers inside the tank, you'll be wasting your time if you've got Oscars. The best thermometers I have used are external and therefore out of the reach of destructive Oscars. You don't have to pay a fortune, you can get one for £10 and it will last you for years.

Aquarium lighting

Oscars do not require any specialised lighting, in fact, you could probably leave the lights off all day and it wouldn't bother them in the slightest.

However, we have all gone to the trouble of setting up an aquarium so it's obviously nice to be able to see what's going on. It's even nicer if you can use lighting that brings out the colours in your fish, although many Oscars are rather dull and drab. Having said that, lots of tropical fish do have beautiful colours that can be wasted if the aquarium lighting isn't very good.

use the Kelvin rating, this is basically the colour temperature of the light, the full spectrum of light produced by the sun which consists of different wavelengths. I know, pretty boring and not really that interesting to talk about unless you are an aquarium lights enthusiast.

The best advice I can give you is to think about what type of lighting you want to achieve. Most people want to see their fish properly, maybe the colours popping. Therefore depending on how many bulbs your system takes, you could opt for one Marine blue and one Marine white. Maybe you've got a large aquarium which takes three bulbs, you could opt for two Marine blue and one Marine white, or vice versa. It's impossible for me to tell you exactly what you need, the best thing to do is go to the fish store and see what's available, I'm sure they could give you some really good advice. Alternatively, come and talk to us on the forum and somebody will be happy to advise you on lighting.

Limiting the time your aquarium is illuminated

One thing I really hate seeing are aquariums that are left illuminated 24 hours a day. In the wild, the fish will know exactly when it's day and night. Contrary to some people's belief fish do actually sleep. Some people think that because Oscars don't close their eyes they don't sleep... In the tropics where OscarFish comes from you will get about 12 hours of sunlight. This is the maximum we would recommend you leave your lights on for in a 24-hour period. Remember that bright aquarium lights will encourage algae growth so limiting lighting times will help keep your aquarium clean.

Sand and gravel

I think it's nice to have a substrate in your aquarium. I've had both sand and gravel over the years, however sand is definitely my preference. Your fish store will probably stock various grades of sand and gravel that come in all different colours. However, if you just want a standard sand or gravel then the cheapest option is to go to your garden centre where they normally sell large bags of gravel or sand for about £10. Read my article on using sand for your aquarium substrate. A lot of the information in the article can also apply to gravel as well.

Aquarium wood

Wood is a great addition to an aquarium, especially the great big pieces that you can get. However, wood isn't always cheap, especially if you want a really big piece. You can pay over £100 for the really large pieces of wood. But in my opinion, they really do look the business. It's important that you be extra careful when deciding on adding wood to your aquarium. Don't put wood that you find on the beach, or in the woods in your aquarium because it probably won't be cured properly and the chances are it will just go rotten once submerged under the water. My advice would be to buy properly preserved wood that is completely ready to go straight into an aquarium. Be aware that some types of bogwood can stain the water and turn it a yellowish in colour. Some people like to boil wood before putting it in their aquarium so that if there is anything nasty present they can eliminate it. When you first put wood in your tank you may find it floats. Don't worry about this, it will soon become saturated and sink to the bottom. Alternatively, you could always attach a few weights to make sure that it sits properly on the bottom of the tank. I like to place my wood near the back of the tank, rather than too close to the glass. You'll find that fish such as plecs, loach, catfish and smaller tank mates appreciate the extra shelter and protection that large pieces of wood offer.


I have tried plants on many occasions, I know other people who have tried planting up their Oscar aquarium. The majority of us have failed miserably because it is quite obvious that Oscars have a devious streak when it comes to real plants in their aquarium. I'm quite sure that Oscars get a kick out of ripping real plants to pieces, it's almost as if they are trying to tell you that they will decide what goes in their home, not you!.

Instead of wasting your money on expensive aquarium plants, why not try some artificial plants? Nowadays they make some very realistic looking aquarium plants that are very difficult to tell from the real thing. The chances are the Oscars will still uproot them, but at least you can put them back into the substrate to live another day.

Now we've discussed the aquarium, filtration and how to initially set the aquarium up we will move on to some other important factors that you will need to know.

Why you must cycle your aquarium

If you don't read anything else on this page then please read this section because it is very important indeed. If you've never kept fish in an aquarium before then you will be unaware of how important it is to prepare the tank properly before you add your fish. I have written what I hope is an easy to understand article on how to cycle an aquarium properly.

Your aquarium water has turned cloudy

It's not uncommon for aquarium water to become cloudy or milky in colour. In most cases, it's simply a slight unbalance in the water chemistry. Normally it's nothing to get too worried about, however, you mustn't ignore cloudy aquarium water because there's always a chance that your fish could be put that risk if you don't do anything about it. I have written a very popular article on how to deal with cloudy aquarium water.

Your tank is ready for fish

Once your aquarium is cycled and you are ready to start adding your fish it's important that you don't get carried away and put all your fish in at once. We recommend that every fish keeper tests their water on a regular basis. However, in the first few weeks that your tank has been set up the filtration will be quite delicate so it's important that you keep on top of your water testing to make sure that water quality doesn't degrade.

If in doubt test your water

It's important that every fish keeper has the means to test the aquarium water. You can take a sample of your aquarium water to the fish store where they are normally quite happy to test it for free. However, you have to bear in mind that things can go wrong at any time. Therefore, I would recommend that you get yourself a master water testing kit which will enable you to test for ammonia, nitrite, nitrate and pH. Understanding how these dangerous toxins can affect the fish and how you test for them will benefit you in the long run.