I've been wondering exactly the same thing about my zebra danios. I have noticed they are incredibly greedy and get to all the food first. They could be constipated depending on what you are feeding them. I feed mine peas once or twice a week as this can help if they are constipated. I just blanch the peas in a bowl with boiling water from the kettle, then de-shell them and crumble them up in the tank. Most of my fish eat seem to love them.