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 there are others you could consider. I would avoid cockatoo which are fairly popular but extremely dull (boring) fish.
I'll second the Apisto recommendation.
I had a pair of Redline who did rather well in my 20 gallon, before moving on to a tank of their own, where they are now apparently breeding.
I swapped those for three Cockatoo, (two female and a male) and these have also settled in nicely, with one smaller (younger) female laying. I'd struggle to call these 'boring'.
Granted, I am yet to see them execute a decent Paso Doble and his card tricks leave something to be desired, but still...
I appreciate that some fishkeepers do view aggression as entertainment, (present company excepted, I'm sure

), but these would even fail in that regard.