One day a few weeks ago, I woke up to find a "trench" the fish had apparently dug overnight. It was along the short side of the tank and he had built a little mound of sand. When I did my normal vac/water change, I smoothed it out again. But now he's done it again along the side and a little along the front of the tank. I don't really care if that's how he likes it. He must do it during the night because I've never seen him do anything to move the sand during the day. Could that be his sleeping space (never caught him "sleeping")? He seems completely normal/healthy so I don't think it's any health problem but I wondered if anyone else's bettas are diggers?