Why are my Goldfish eating rocks??

Goldfish are scavengers and its their nature to keep looking for food, which also kills them because they don't know when to stop. They just suck on the pebbles to look for more food, so DON'T overfeed them because they will eat left over food too. Only feed as much as they can eat in 2 mins and twice a day. Overfeed them causes constipation and swim bladder disease.