I was wondering if my axolotls are fat, or if they should look this way. Their bellies seem very black when i look at them (not on the picture). Could this be from if they are eating the sand? The sand was the finest one i could find, would it be any problem for the axolotls to pass them? I see them sometimes eating the sand but they usually spits it out. Is it bad for them to eat sand even if they pass it?