GFO removes phosphates.

GAC removes impurities skimming can't. It will also 'polish' the water and make it clear(er).

I will say you definitely are better off starting with these things, GFO at least, before you accumulate phosphates. It's much easier to prevent algae from the start than to get rid of it later.

I have the BRS dual reactor, works well. Although, I kind of wish they were separate.