How much algae do tangs eat??

I cannot agree with you anymore. Lawnmowers are excellent algae grazers. As a matter of fact, my lawnmower is the primary clean up crew member; he keeps my rock spiffy clean! I also dose some iron (safe amount) to promote a little hair algae growth to keep him with a steady amount of food. I monitor the amount of algae growing in my tank by looking at the size of his belly.