Fish that thrive in similar water chemistry and similar temperatures get along swimmingly.
1. African cichlids
These active fish can be territorial and aggressive. Make sure your tank has plenty of space and decorations, so each African cichlid can find a spot to call its own.
2. South American cichlids
These fish aren’t as aggressive as African cichlids. Still, they get along best with each other (and fish of other species) if they’re all about the same size. Keep in mind that South American cichlids can grow to be anywhere from six to 12 inches long.
Goldfish are laid-back, easygoing and hardy; they can live up to 20 years. They require lots of space, are big eaters, and produce lots of waste.
4. Tropical fish
These active fish are compatible with other non-aggressive species of fish. Schooling species do best in quantities of five or more.
5. Semi-aggressive tropical fish
As their name suggests, these may bully smaller tank-mates, so aim for an aquarium of fish all close to the same size.