20G is probably bare minumum for a single fish, if you get a pair, at least a 55g
they get up to around 12 inches, so a 20 long might be kinda tight in the long run. a
30g or 40g breeder tank is excellent because they are long and shallow, the depth of the water is irrelevent to a 'skipper
Agree with Shikkapow, I like the 30 B or 40 B tanks. You have extra width to work with the arrangement of the tank. Also, some species are larger than the others. HTH