I have a small adult female in a 33 gallon. That would be the smallest tank I would keep a JD in. Even that is tight. But she is small for an adult JD.
Some SA's that would go nicely in the 20 gallon are
Just about any of the Apistogramma genus. They are peaceful colorful cichlids. They are very hardy for little fish and are quite undemanding.
Others like Cleithracara maronii. Keyhole cichlid. Nice peaceful fish.
The microgeophagus genus. Rams are great small tank fish.
Some SA's that would go nicely in the 20 gallon are
Just about any of the Apistogramma genus. They are peaceful colorful cichlids. They are very hardy for little fish and are quite undemanding.
Others like Cleithracara maronii. Keyhole cichlid. Nice peaceful fish.
The microgeophagus genus. Rams are great small tank fish.