I just got a great deal on a 20 gal tall and a colony of approx. 30 N. Brichardi, for $25. I have not kept this species before and I was wondering what is considered a good number of these fish for a 20 gal tall?
They would be better in a 29G 30" long tank and buy six to end up with a pair. That pair will try and may succeed to kill everyone else in the tank. And then they will fill the tank with their fry and tolerate them.
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.