I have a small problem with my trigger fish, was wondering if it was common with this type of fish. He is very healthy have had him over two years now, but lately he has been digging up the sand all the way to the glass. there is one rock he really likes and everytime I fill in more sand around the rock within a day he has done it again. I have added a brown tang to the mix about a month ago. Any way to stop him from digging?