a 20 long.
one question. why don't you use platies or mollies? they need less room, and produce offspring twice as large as convict fry. also, in a 20 gallon tank, you could have quite the breeding colony.
my problems with feeding cons are that some may survive and go on to conquer your tank, and fish may choke because when the cons are swallowed, they stick up the spines on their back and get caught in the diner's throat (ouch)
just my 2 cents