I think I'm reading that you currently have 30 fish from an established breeding colony in the 20 gallon- is that correct?
If so, it's time to thin the herd. Net out a bunch of fish in the 1-2 inch range and bring them to your lfs for cash or trade. Make sure to leave the breeding couple, though- so maybe watch them for a week or two to see who's at the top of the pecking order. In the mean time, you are going to need to do water changes pretty frequently to keep that many fish in that small tank. Changing out 10 gallons twice a week should keep them healthy.