I suspect you measured the exterior and this is actually a 20 high. I would not keep keyholes in a 20 high. You might keep a pair of borelli (a very colourful personality fish); but if you keep a pair (female/male or trio female/female/male); you would need to well landscape it so the male can hide from the females with frys. You could try german blue rams if you have a fair amount of experience as they can be a bit delicate.
Apisto in general are very hearty but if you keep pairs of trio (males with females); depending on the specific species special care need to be taken. If you keep just male or males things are a bit easier. I mentioned borelli above because they are a bit smaller and adaptable to wide range of water parameters but their are others you could consider. I would avoid cockatoo which are fairly popular but extremely dull (boring) fish.