A 20 gallon tall tank, at least in the standard configuration, has exactly the same footprint (i.e. floorspace and surface area) as a 10 gallon tank. Thus it is a bit too small for most cichlids. You could try a pair of kribensis or Bolivian rams, with some small tetras for the midwater and upper layers. You will need to keep up on your water changes to keep those fish happy, however.
The electric yellow lab you mentioned would probably do OK in the tank, but only if you were just going to keep the one fish. More than one would be too much of a load for the tank to handle long term, both from a fish waste standpoint and from the issue of territoriality as well.
Another option would be to go with some of the very small shelldwellers from Lake Tanganyika, but they will only inhabit the bottom of the tank and require pretty specialized water conditions to thrive as well.