What is a good mix for fresh water fish in a 20 gallon tank?

In a properly set up 20 gallon tank you will have a maximum of 17 gallons of water when it is filled to within 1" of the top and has gravel substrata for plants to grow and rocks for decore/aquascaping etc. The rule is 1" of fish needs a minimum of 1 gallon of water. So if you wish to have a "Tropical community tank" you could be including fish from any or all of the following species. Tetras, DaniOS, Barbs, Rasboras, Corydoras, Anabantids, Dwarf cichlids and many other types of fish too. Just remember not to overstock and do at least a 50% water change weekly. Good luck and enjoy your fish.