Full Guide to Oscar Fish Tank Setup including Size & Decoration

Last Updated on May 27, 2019

For those of you who are thinking about buying an Oscar, or for those of you who have impulsively bought an Oscar, this article will go over some basic pointers about setting up an aquarium for your new friend. Oscars are a serious commitment due to their longevity and adult size. Having the correct set up will make their transition from little wriggler to a big brute as seamless as possible.

Oscar Tank Size

There is plenty of debate about the minimum size for an Oscar Tank. I believe that the smallest aquarium an Oscar should be kept in is a 55 gallon (48x12x21 inches). This is not ideal, but it can be done. In a 55 gallon tank I can only recommend one Oscar with no tank mates and minimal decorations. This will also mean that you will be performing large frequent water changes to keep nitrates at a safe level for your Oscar.

The average Oscar will grow to 12-14 inches in length. However there are some beasts that will grow up to 15 even 16 inches long. For these beasts I recommend a 75g tank. Let’s not forget that bigger is better, and it seems that today there are more and more people providing 100 + gallon aquariums for their fishes. Don’t be afraid to give them some extra space. It will be much appreciated.

One Oscar

Most aquarists agree that a 55 US gallon tank is the absolute minimum tank size to house an adult oscar. This is because a smaller tank simply does not have enough water in it to dillute the waste produced by the oscar. It is also because a smaller tank simply does not have enough room for the oscar to swim. A standard 55 gal tank is about 4 feet (122 cm) long and one foot (30 cm) wide. Since an average Oscar grows to about 12 inches (30 cm), a 55 gal barely provides room to turn, and provides a straight line swimming distance of about 4 body lengths, which is not much.

If you already have a 55 gal tank, then fine, it will probably work for your oscar by himself. However, if you are buying a tank from scratch, I strongly recommend that you get a 75 gallon (286 liter, 4 feet by 18 inches, by 20 inches) or larger tank. This is for two reasons. First of all, a tank with a single fish in it is somewhat boring to most people. At some point, you will probably be tempted to get more fish. With a 75 gal (or larger) tank, this is a possibility. The second reason is because it is quite possible that you will end up with a larger than average oscar. Oscars can and do grow up to about 18 inches (45 cm). A 55 gallon tank is simply not big enough to house a fish of this size. If you have a 55 gal, and you end up with a monster like this, you will end up needing to buy another larger tank. It is cheaper and more eficient to just buy the bigger tank in advance. When you consider the total cost of a complete set-up, the increase in price of a 75 gal over a 55 gal is not that big of a deal.

Two Oscars

For two oscars, most aquarists agree that a 75 gallon tank is the absolute minimum. However, again, if you are buying a tank, I strongly recommend that you go larger. A 125 gal (476 liters, 6 feet by 18 inches by 22 inches) or larger tank will greatly increase the odds that your two oscars will get along, and will allow you to possibly have some other fish. You also have to remember, minimum size equals maximum effort. In the case of two adult Oscars in a 75 gallon tank, you need to plan on bi-weekly large water changes.

Three or more oscars

A 125 gal tank is the minimum for 3 Oscars. Again, if you can, go larger. I would recommend 150 gallons for three Oscars to limit your water change requirements to one per week. But since three Oscars is never a good idea, might as well go for the 200 gallons and get 4. Cool

Other Stocking Considerations

Please keep in mind, “absolutely minimum” requires maximum effort and as tank size decreases, so do your chances of long term success with your fish. As an example, while you can be successful with 2 Oscars in a 75 gallon tank, I would not want to attempt it in anything smaller than a 6′ 125 gallon tank. The added tank length creates additional territory which reduces the chances of aggression issues while the additional water volume reduces maintenance requirements. Two Oscars in a 75 gallon tank will require at least two large water changes per week. Two Oscars in a 125 gallon tank will require one moderate water change per week or possibly one large water change every two weeks. If you plan to have other fish in your tank besides oscars

Oscar Tank Filtration

I’m not going to get into the specifics of filtration and bioload here. Just know that Oscars are very messy fish and bring with them a large bioload. In most cases I recommend canister filters in conjunction with a HOB (Hang on Back) filter. That way if one filter goes down you have a back up. One thing is an absolute, unlike community fish that many new aquarist are familiar with, an Oscar Tank requires massive filtration with a focus on biofiltration. The more the better. If using HOB filters we recommend enough filtration to turn over the entire volume of the tank 10 times per hour. If using Canister Filters, our recommendation is enough filtration to turn over the tank volume four times per hour.

Oscar Fish Tank Decorations

In general, adult Oscars prefer swimming space. A tank that is packed full of decorations is no and ideal Oscar Tank. They like to be able to see potential threats coming so they can face them head on. Juvenile Oscars however, do enjoy some type of hiding spot so you may have to accommodate them as they grow. Using a large terracotta pot or a cave decoration can make a shy baby Oscar feel more comfortable. But as they mature they will become more aggressive and will not need or desire to hide as much.

Oscars are terribly clumsy fish. Couple that with the fact that we often keep them in small tanks and you’ve got a big bumbling brute in a jewelry store. Any decoration, rock, or driftwood with a sharp edge is a no-no. Oscars can be quite jumpy and if spooked they will run into whatever is in front of them. This can lead to scrapes, gashes, even damaged eyes. Use larger sized rounded rocks that cannot be moved, or driftwood that is sanded down. Most decorations you find at the pet store are safe as well as long as they are not pointed.

Don’t lean things upon each other. Stacking rocks is not recommended. Oscars are big powerful fish and can tip over such things that may lead to a disaster. Live plants can be tried with Oscars though as with most cichlids, they will most likely be uprooted frequently or just plain shredded to bits. IMO live plants are worth a shot because they can really add color and something else nice to look at. But don’t expect them to survive or you will be disappointed more often than not. More about decorations here.

What to avoid

When setting up an oscar tank, a few things need to be kept in mind.

First, Oscars love to redecorate. If it is small enough for the oscars to move, it probably will be moved at one point or another. This rules out anything that is remotely breakable, such as ceramic ornamentation. It also rules out most plants (real or fake), since they will be killed/destroyed over time by the oscars. Fake plants can be used, but don’t get elaborate in their use because whatever concepts you have in decorating your tank, the Oscar will have his or her own, and anything you do will eventually be undone by the Oscar.

Another consideration is that oscars move around a lot and are fairly clumsy. This means that they bump into things a lot. That rules out anything with sharp edges and/or rough textures, such as lava rock. Oscars have a habit of scratching themselves up or even ripping out large chunks of flesh, either of which, for a fish, can become life threatening.

Finally, remember that Oscars are softwater fish. Unless your water is extremely soft, you will want to avoid anything that will tend to raise the pH of the water in the tank. This rules out any calcium carbonate-based minerals such as coral, aragonite, limestone, marble, sea sand, and sea shells.

Acceptable Items

So, what does that leave us? A mainstay of oscar tanks is driftwood. The tanins in the driftwood tend to buffer the water to the lower pH range that Oscars prefer. Also, driftwood sold at retail normally is attached to heavy pieces of slate, so it is difficult for the Oscars to move it. Finally, plecos, which are a common bottom feeder in an oscar tank, like to scrape on driftwood.

Another good decoration is large river rocks. They tend to be smooth, so the oscars are not likely to injure themselves if they bump into a rock. They also give the tank more of a natural look.

Finally, every fishtank with egg-laying cichlids should have at least one piece of slate. This gives the fish a flat surface on which to spawn. If they do not have a convienient flat surface for this purpose, you will often find that they have dug spots in the gravel all the way to the glass.

Plastic/Silk plants can be used, preferably those with a weighted base, provided you don’t mind your fish placing them somewhere other than were you intended. Your Oscar will toss these around almost like playtoys. Without a weighted base, most plastic plants in an Oscar tank will spend their time floating at the water surface. Wink

Oscar Tank Tops/ Lids

Oscars are known jumpers. They have been looking ‘up’ for their food for centuries and even though the majority of Oscars we keep are far from their wild ancestors they still look to the surface for food. A lid or some kind of tank top is necessary to prevent your Oscar from jumping. If you bought a used tank and don’t have a lid or a tank top you can fashion one out of some egg crate, or a cut piece of Plexiglas.

Do Not train your oscar to jump out of the water for food. Too many Oscars leap out of their aquariums and either fall to their deaths or lie helpless on the dry floor. Unfortunately, you do not have to train some Oscars to do this, they just will.

If you suspect that your Oscar is a jumper you need to weigh down the lid. Put some books or something heavy on the lids. If you are a neat freak and don’t want a bunch of books or junk sitting on your fish tank, you can buy strips of velcro and use them to fasten your lids down nicely.

Substrate for an Oscar Tank

Again, I won’t go into the finer details about substrates or all of the available substrates for Oscars Tanks. The big debate between substrates is sand vs gravel. Either substrate will suffice for an Oscar. Just remember that Oscars like to dig. Fine sand can work its way up into a filter and ruin it. This is especially true if you have a large Oscar who likes to take a mouthful of sand and ‘spit’ it. Sometimes right into the filter intake. Be sure and use a pre-filter on the filter intake if you have sand or there is a good chance it will be damaged by the sand. More on gravel vs sand here.

You don’t want too much substrate. Especially with gravel, uneaten food and debris will work its way down into the substrate so if I am using gravel I prefer a shallow substrate bed, 1 – 1.5 inches. Sand is more fine grained and doesn’t allow debris and such to work into the bed (not as much anyway) so I like a deeper bed with sand 2 -2.5 inches. Others prefer a shallow sand bed. If left undisturbed, sand will allow gas build up from nitrogenous waste that is potentially dangerous. Sand needs to be stirred up so this gas cannot accumulate.

There are others still that prefer no substrate at all. Bare bottom tanks are becoming more popular due to the ease of maintenance. Some tanks with added water movement achieve a flow that doesn’t even allow debris to settle. In most cases, the flow is directed toward the filter intakes so any debris caught in the flow will eventually be caught in the filter.

Heaters in an Oscar Tank

Heaters might not seem like a noteworthy point to hobbyists who keep small tetras or African cichlids. But for those who have kept large rowdy cichids you know heaters can be broken. Yes, that’s right. Your Oscar can and likely will take a swing at your heater.

A broken heater can cut your Oscar, over heat your tank, or in the worst scenario, electrocute and kill your Oscar. You want a heater that is shock resistant. Pretty much any heater that is glass is a bad idea for an Oscar tank. If you don’t have the availability or already have a glass heater then you need to protect it somehow. You can make a heater guard out of some PVC piping cut in half (the long way) and attach some suction cups to it, or you can fashion a guard out of egg crate.

Bottom line is that you want that heater protected from your big brute. The other point I want to make is that of heater malfunction. I personally have had two Marineland Stealth heaters somehow malfunction and over heat my aquarium. I lost several fish to this and I was severely disappointed. For this reason I cannot suggest these heaters. Stealths are known for being ‘shatter proof’ which is why I bought them. But after my experience I’ll never own another. I currently have several other brands that I’m testing.



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