Welcome to AquariaCentral first off. I hope you learn a lot from all of the kind people on these forums.
The thing is though, an 8-10" fish should not be kept in a 20g. Not even by itself.
Fish should always be given the largest tank possible, and they will in turn, provide you with more enjoyment.
Do you already have a 20g aquarium? If so, there are plenty of interesting setups you can have in a 20g, such as dwarf cichlids, or maybe a small community tank.
Serg