I've had about ten in my 30 gallon for probably 6 months now (they were not all purchased at once). They didn't get rid of the vermetid overnight, but I rarely see any of those annoying snails anymore. If I were you, I would buy a dozen, wait a month, buy another dozen. They will clean up your tank in a few weeks probably. They don't only eat vermetid, so they will do fine in your tank after the snails are gone.Thats not working, or maybe I don't have enough. How many is a good starting number? 90 gallon tank.