Weird behavior for a Koi Fish?

My koi fish is about 6 inches long, not counting the frilly tail part. There are small pebbles in the tank. The koi fish seems to suck the stones into its mouth and then spit them out. Sometimes instantly and other times it will swim a bit up then release the rocks. What is going on?! And what do Bala sharks eat because i hardly see them eat anything.