Small koi fish.

I recently bought a small koi fish from my local Petco.  Everything has been fine but I've noticed at night that I hear clanking noises.  I watched him and saw that he's swallowing the tank gravel and spitting it back out repeatedly. He also has started almost resting behind his coral for extended periods of time.  I feed him regularly and his water temperature is normal. What could be going on with him?

[ChitChatNine]

How is your fish doing as of this writing?  You can always call or stop back to Petco.  They usually are pretty good with helping you sort through questions and if they don't know they have a vet that stops by and they can ask and get back to you.

C~