My only suggestion, and this is a guess because I have never had the problem, is that as long as they are at least 4-5" long, reduce how often you feed them to every other day, once aday if smaller, and leave the tank light off while doing so. When you feed them drop in one morsel at a time and only add another after they eat that. Once one starts to feed the others will too. They will eventually start feeding competitively.