- Submitted by Saluki, edits by Kmuda
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.
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.
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 gallon and get 4.
Please keep in mind, “absolutely minimum” requires maxium 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, please refer to How can I tell if my tank is overstocked?.