I Appreciate what gupppies and Parttimer have to say about cichlids and tetras. That mouth size factor again comes into play.

So also does the behavior of cichlids. As a rule, tetras would go well with the dwarf, South American cichlids. But when even the innocuous Nannacara anomala spawns, the little one inch females will chase their far larger males to a far off corner of the aquarium. They will proceed to murder every Cory in the tank, lest they eat the cichlid's eggs in her nest. She even changes from demure brown hues to a bold checkboard pattern (which her fry will identify with and follow - they've done some interesting experiments on that pattern.) When other fish see that pattern, they "know" they had better swim to the other side of the pond or die at the hands (fins? mouth?) of the most ferocious 1 inch fish they will ever encounter.

Cichlid people keep "companion fish" such as rainbowfish, which swim around in the open water and help the cichlids to feel comfortable and safe. The cichlids then venture out where they can be seen in that aquarium.

Cichlid people also speak of dither fish - like some tetras and livebearers. The purpose of the dither fish is much the same as the companion fish. Their fate though, may not be the same. No one uses expensive fish as dithers. :(

Angelfish, severum, festivum (whatever their scientific names are today) come from similar reedy backwaters (hense their design). In a 55-gallon or larger tank, several of them, with good water quality, can be kept together. If you research comparable sized fish, their behaviors, diet and habitat, you can find fish which can be kept together in an appropriately sized aquarium - though they might be pretty big.

Ten-gallon tanks are often sold as beginner tanks. In many ways 20-gallon tanks would be better. While bigger isn't always better, you will find you can do even more with that new tank of yours.

Tetras also belong to a larger group of fishes called the Characins (care - a - sins). These include the piranhas, a medium sized bucktoothed tetra, Exodon paradoxis, which will tear up the larger piranha and much larger fish-eating characins which are also popular game fish. You might welcome them to the dinner table, but not usually in your aquarium.

They even sell little pacu in the shops. I recently saw some of those at two feet long. Though they are vegetarians, they have jaws which can crack nuts and probably fingers.

So checking around is essential. Do what we may, some fishes are only going to be successfully kept in a species tank or by themselves.

About a year ago, a GL member (GuppyAddict?) mentioned a web site which listed compatible sets of fish. We immediately disagreed with some of their combinations, but the site was a good start. Several books will offer combinations too.

Also do a Google search for cichlid clubs or Canadian cichlid clubs. Their sites may also offer insights into those most studied fishes.