They are prolific but they are also self limiting. If you never remove any fish, their numbers will stabilize at about 3 per gallon or maybe slightly more. It is the way I have maintained the numbers in my 20 gallon for a couple of years now. If you want to sell endlers at an auction, try picking out the nicest ones you have and sell either 2 pairs or a trio. For some reason, they sell much better like that than they do if I bag up 15 or 20 together. If I take those 20 and just bag the best pair, it will return more money at an auction.