20 GAL is suitable for fry or refugium, maybe a temporary hospital tank. It all depends on what you really WANT to keep for cichlids. I have seen some Tanganyikan dwarf shell dwellers in such a tank, but it didn't feel right. I had shell dwellers in a 75 gallon so i might be a bit biased. Lamprologus ocellatus is a safe choice for short term, if you have a breeding pair that needs privacy. They like to grab the shells in their mouths and run them all over the tank, so wide open sand spaces would be the preferred environment. For the south/central american crowd, they like some nice dwarfs that MIGHT be ok in a 20.
Just bear this in mind, tiny tank for tiny fish, big-ass tank for big-ass fish. Pardon my french. Hope this helps, hope others have some useful input as well.
