yeah follow other advice given, and i would up it to at least 50% a week and see if that helps, what are your water parameters between changes? and try to watch your tank closer and see them poo because if they have parasites, the entire tank will need to be treated before any of the fish are moved to another tank, even when your not having problems its always a good idea to closely observe your fish at least once a week so you know where they all stand and can catch problems earlier right as they may arise