Is he eating other foods? If he's eating other stuff, he may not have figured out the algae wafers are food--maybe try dropping in a quarter tab on a nightly basis until he works it out and vacumming it up if it's still there in the morning. If he's not eating anything....there are a lot of possibilities. Is he a new fish? He may be settling in, or if he's by himself, he may be lonely (or both). If you've had him a while and this is a new behavior, he may be getting sick, or something's changed in your water, maybe?