My naighbour had this problem when he first got his fish...turned out that he was over feeding them. This sounds stupid I know but apparently there is something in the food which releases into the water and kills them. The pump is usually able to filter out this bad stuff but if you are over feeding them then the pump may be over worked. Not sure how to tell if you are over feeding them though as I don't have fish myself. Mabie the link that other guy gave might help. Hope I helped.