I'll disagree with you stoneage on 20 gallon (long) being too small for a pair of convicts. Granted, it would be better to have a bigger tank, but lots of folks breed convicts and keep pairs in 20 gallon tanks. A 6" fish is 1/2 the width of a 20 gallon tank which is a fairly reasonable ratio - especially since the length is 5 times the fish's size.
Eric
Eric