In my experience they like to have lots of room to swim, they are spastic fish...i would go larger than a 20. Maybe a 30, 38, or a 40 gallon breeder tank.  those are bigger than the 20, and you could add maybe a couple more.  they do like to swim lots..so i dont think 20 is big enough.  and dont try to cylce your tank with them...ive seen bad things happen with that...