A 20 gallon tank can easily sustain 40 fry if you are careful about water changes. That does not mean that you can simply cross your fingers and hope for the best though. Instead, you will need to provide minimum maintenance on your tank. I have found that a population that is a bit heavy in juveniles will bounce right back if I sell off all of the adult fish. What that means to me, as a breeder, is that by selling all of my mature fish every few months I can expect to have great survival rates with new fish while retaining no mature fish in my stocks. I am not a commercial breeder of fish so having a continuing interest in a particular fish means that I have failed to pass on the fish in my care. I like to sell my stock by merely selling any surplus fish. The end result, as I see it, is that I retain fish that are young and can be expected to produce fry for quite some time. I sell my surplus fish with no reservations and do not hold back the fish being offered. That means that buyers often can get a good deal on my fish and that I can often do better than a typical vendor would do with retail markets being what they are.
