I know it contains a phosphate and nitrate sponge, as well as other things. It advertises it's "chemical free" which is the biggest load of marketing BS ever, oh well... c'est la vie.
It also absorbs a lot of things that will inhibit plant growth. Light-stemmed, fast growing plants (like cabomba and hygro) have been known to be starved to death by this stuff. I'll stick to a balanced tank to keep algae away... might be tempted if I didn't have plants. I've never heard of it causing trouble in a non-planted tank.