Yes they can and do survive off of algae if there is enough of it. Did you transfer him to a new tank or is it established? He might not appreciate the lack of algae in a new tank. I know of someone who has a tank set aside for the sole purpose of growing algae for these guys to eat. They transfer round river stones from the main tank to the algae tank until it is covered, then put it back in for the fish to clean off. That way the ideal conditions for growing algae can be the only purpose of the extra tank. I cant link the thread he documents it because its on another forum but if you're interested you can PM me. Its very interesting. Maybe one day I'll set up a tank for some, but for now I've got my botia keeping me happy.
--Tammy