I've got a good sized Dempsey (approximately 6 inches or so) who lives alone in a 55g tank. He's taken the last 2-3 days to sulking behind a rock under the filter intake. He's still alive but won't come out for anything. Normally, I walk in the room and he comes out hoping to be fed. Lately, even when I put food in, he won't come out. He just sits there. Is there something wrong with him? I know the water is fine as I just did a 20% water change.