They will eat it, although extremely slow IME. If you have a large tank, they aren't going to do much. They also have difficulty holding onto rocks and such in the high current in most of our tanks. I added 6 to my 210 a while back, and they never really made a dent. A waste of money in my case. I tried sea hares that chowed on it, but it grew back faster than they could eat it. What really did it for me, was raising my mag to 1600+ and dosing sugar, probably attribute more of it to the elevated mag though. I suppose if I got rid of a few fish, that would help also.