Carbon dosing and gfo are the best way. Ats are pretty weak at removing much of anything and the sam can be said for a refugium with a little ball chaeto. Most of the refugiums you see hobbyist use are small Little sections in there sump with a small Bundle of chaeto, in order for a fuge to really be effective if needs to be a whole lot bigger then what most people use. If you really want a clean system you need a good skimmer and some form of carbon dosing along gfo and water changes.
Don't get me wrong, you can have a successful Sps tank with out carbon dosing but most of the
Time you will be battling po4 and nitrates often.