The oscar is a common choice for people looking for a large predatory fish. It is a member of the cichlid family known for their big personalities and aggressive nature. Many people enjoy watching their oscar make a meal of the poor feeder fish that swim too close. It's hard to resist the big eyes of a baby oscar begging you to take him home, but they make a poor beginner fish for several reasons. First of all, they require a larger aquarium than most are willing to start with. oscars can grow to be up to eighteen inches in length, and they have a large body mass. The minimum would be a 55 gallon for a single adult oscar, but even that is pushing it. You would have to be meticulous in doing large weekly water changes and have no other fish in the aquarium. Because of it's size and messiness, the oscar really should be kept in 75 gallon tanks or higher.
Coldwater fish include goldfish and minnows
Betta fish can live in community tanks, under the right circumstances. In fact, under the best of circumstances your Betta will have far better quality of life in a community tank than he would have in a tank by himself.
Fish Tanks & Aquariums | PetSmart
Large volumes of water enable more stability in a tank by diluting effects from death or contamination events that push an aquarium away from equilibrium. The bigger the tank, the easier such a is to absorb, because the effects of that event are diluted. For example, the death of the only fish in an 11-litre (3 US gal; 2 imp gal) tank causes dramatic changes in the system, while the death of that same fish in a 400-litre (110 US gal; 88 imp gal) tank with many other fish in it represents only a minor change. For this reason, hobbyists often favor larger tanks, as they require less attention.
Aquarium Water Care for Healthy Fish Tanks | PetSmart