i use Corallife activated carbon(it claims to be PO4free,never tested it).1 tbls per 40 gallons is a good start( i only use 2 tsp).After rinsing the catbon well,let it soak in a glass of RO water for about an hr.Fresh carbon can delpete oxygen levels in your tank thats why i like to soak it first.The first week or so,i'd probably use it passively with decnt water flow around the carbon sock in sump.After that,I;d run it active through a filter sock placed under the output from the tank(or a dedicated carbon filter).Be sparing(especially in the beginning) with it when using it active as to not shock the tank,I'd use 2-3 tbls and change it weekly.