My big plec does that aswell, all over the tank, he doesn't seem so bad now, I think he was after a nice place to hide, and my coconut caves were rather on the small side, so now he's got a resin root sort of thing, which isn't the naturalist of things, but it saves me having to smooth out the sand and replant all the time.i have a sand substrate at the front of my tank now and he digs right down to the bottom glass all the time (quite amusing to watch, thank god i've no forground plants). with gravel he never touched a pebble though. if you've got a gravel substrate you'll be fine.