Oscar cichlids are definitely one of the most popular large cichlids available in the hobby. Their size, coloration, and personality make them an attractive first cichlid. Although popular, most Oscars are not properly cared for.
The biggest issue is their size. Unfortunately most end up in a 55 gallon tank. This will result in stunted growth. Because this is so common it is a very common myth that they only get to 10-12″. This is really just a self-fulfilling prophecy. If everyone thinks they are okay in a 55, most people put them in one, meaning most Oscars are stunted, which serves as ‘proof’ to all those who think they don’t need anything more than a 55. The truth is that they can get larger than 12″, much larger. I have personally seen Oscars that were at least a full 15″. I have heard and believe reports of them as large as 18-20″. FishBase.org lists them at a full 18″. These are not small fish. Although a 55 may be okay for a while, they should be upgraded no later than 8-10″ (usually about one year). Long term a 75 is the bare minimum for a pair of Oscars. Although I would say a 75 is the bare minimum for one Oscar, assuming they get along and the tank is maintained properly 2 can fit very well in a 75.
Head and Lateral Line Erosion (HLLE):
Oscars are exceptionally sensitive to Head and Lateral Line Erosion (HLLE). Many if not most people think of it as Hole in the Head (HITH), but I firmly believe there is ample evidence showing the two are not one and the same. HITH is best limited to parasitic infections of Hexamita spp. and Spironucleus vortens, both typical in Discus and Angels. Oscars on the other hand get HLLE which is not parasitic and is caused by low quality food and water. Big weekly water changes are essential, ideally at least 50%. They need to enough to keep the nitrate under 20ppm or within 10ppm of the tap. For more information see diet below and the article on Hole in the Head (HITH) and Head and Lateral Line Erosion (HLLE).
Diet is just as important as water quality. Oscars should be fed high quality pellets exclusively. I only feed and recommend New Life Spectrum. Live foods are not essential or even beneficial. They are nutritionally incomplete, unbalanced, and in all likelihood will introduce pathogens sooner or later. They also increase aggression. This isn’t to say they turn Oscars in to blood thirsty fiends, but if you have two tanks of Oscars, one fed live foods (even just occasionally) and the other not, there will be a difference. You will be more likely to have issues if they are ever fed live foods.
When small they can be fed as much as six times a day. Feed as much as they will eat in about 5 minutes. In nature the little guys would be eating as much as they can as fast as they can find it. They need to grow as fast as they can so that they are large enough to no longer be an easy meal for half the fish in the Amazon. As they get larger their growth rate will slow and their feeding behavior will shift from many small meals to fewer, larger meals. By the time they are full grown they may only need one or two large meals per week (preferably a day or two before the water change).
Filtration plays an important role in any tank, but Oscars can certainly test filters to their limits. These are big, messy fish. The best filtration is a large, powerful canister filter. I prefer Fluvals. Given the minimum tank size of a 75 I would just go straight to the FX6, but a 406 may do if it is just a 75 and you have some additional flow (ideally from a strong air stone). The problem with canisters is that they are a pain to clean, which means most people neglect them. In the end this makes them not as good for the fish. If you go with a canister it is essential that you clean them well every single month. None of this ‘they are fine until the flow is reduced’ mythology some lazy aquarists like to spout.
Tankmates for Oscars can get tricky. They are large so certainly anything that can fit in their mouth will be eaten. However, in the world of big cichlids Oscars are actually big babies and can easily end up at the bottom of the pecking order, even with much smaller cichlids. I have seen 2-3″ convict cichlids push around 10-12″ Oscars. So in general I would not recommend them with other big cichlids because the others are too aggressive (Jack Dempsey, red devil, managuense, etc.). I also don’t like the use of dither fish such as giant danios being used to give the Oscars a target to chase. Even if they never catch them, that is not an acceptable home for small fish to be stressed like that. The best tankmates for Oscars tend to be large semi-aggressive fish that are too large to be eaten or beaten up by the Oscars, but are not aggressive enough to bother the Oscars. Large catfish, large plecoes, oddballs like bichirs, and medium cichlids in the 6-10″ range are the best options. Obviously tank size and layout, water changes, and individual personalities will affect the exact options.
No matter what you decide to try as a tankmate, ideally they all grow up together. If Oscars grow up with other fish they will be much less likely to view them as food or intruders to their territory later.
Although I usually recommend sand, Oscars are one of few exceptions. Their size and eagerness to dig around make it too likely to cause problems with the sand, mainly getting in the filter. I still don’t recommend a normal sized gravel, instead go with Bits O’ Walnut. They are making it again. It is a fine gravel and a very nice, natural dark brown color. Although it is finer, make sure to vacuum well with the weekly water change.
Try what you like. Be aware Oscars are known to rearrange as they see fit, so even that 8 pound rock you like having front and center may not stay there. You can try fake plants, driftwood, etc. If you don’t mind the look they love things like PVC connectors. Make sure any rocks or other heavy decor is firmly on the bottom, don’t stack them in a way that would allow the Oscars to dig out from under them and cause the pile to collapse.
I only recommend shatterproof thermal plastic heaters (Cobalt Neo-Therm and Aqueon Pro), but it is worth noting this is especially important with Oscars because they are very capable of playing with heaters, beating them around, and breaking them if they are made of glass.
A True Pair:
If you would like to have an actual pair of Oscars the best way is to get 6 or more juveniles, grow them out together, and wait for a pair to form naturally. With 6 or more you are effectively guaranteed to have at least one male and one female who will figure things out on their if given the chance. Once you have your actual pair find homes for the rest. It is best to get this figured out before you buy 6 and commit to providing them with good homes. In most cases this will be donating them to your local fish store to sell at a cheap price to someone who needs Oscars that aren’t tiny anymore.