You've probably been giving them too much. What you give them should be gone before 24 hours if you're feeding once a day. Try giving less - you'll be able to work out the right amount with some experience and trial and error. When they get larger, they don't need food everyday - although I always feed mine a little daily.
I finally figured out that it could in fact be grated even when it is pretty mushy. When I started grating it, I had less waste as they could all easily get access to a surface.