This post is atypical of the type that I usually publish because I'm generally loathe to be critical of the choices of others. However, I'm going to get on the stump on this one.
Astronotus ocellatus, otherwise known as the Oscar or Tiger Oscar, is a species endemic to South America. It's a very attractive cichlid with many color variations and its popularity in the hobby is very high. However, this is a fish that should be left at the store, unless you're an experienced cichlid keeper and you have a large tank.
Oscars are many things, but one thing they aren't is an easy fish to keep. I don't have anything against the species but I do believe its popularity is unfortunate as it too often ends up in tanks of novices. As an avid reader of fish posts on various forums and cichlid groups, the majority of posts I see about Oscars are from keepers who are asking for help with either health issues or aggression, or are upset that their Oscar ate one of its tankmates.
This species gets quite large and it grows fast. It requires a very large tank. In fact, I wouldn't keep one in less than a standard 150 gallon. Ideally, this species, especially if you keep more than one, begs for something larger.
It's a very belligerent species with a voracious appetite. It will eat pretty much anything it can fit in its mouth, which is quite large for its size. Juvenile Oscars are just as much on the menu as anything else. If it's not trying to consume your other fish, it's pushing its tankmates around. It may not be the most aggressive cichlid, but it's aggression it often under appreciated. It's also a messy fish. Because it eats a lot, it produces a lot of waste. However, big fish produce more waste than small fish, so that's nothing surprising.
In addition, this is too frequently one of the species that I refer to in my previous post. Many cichlid keepers are ill prepared for them. All too often, Oscars end up in tanks that are too small, they get stressed, and they develop problems such as Hexamita, commonly called hole-in-the-head disease. IMO, this cichlid is best left at the store, unless you're an experienced cichlid keeper and you have a really large tank for it.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub