I would invest in a top notch testing kit like a salifert for your phosphates test. My api always says I have no phosphates but the salifert can pick them up and its quite clear what the reading is.
Theres also the electronic version of the test that Hanna makes. I use that now, you put water in a tube, hit a button, then put in a powder packet and put it in the machine. Then voila 3 min later you have an exact number. I think I paid $30 for my kit.
But back to the question. lol If you have any phosphates it should be addressed and a reactor can do that. These are nicer than the phosban and have great reviews and they are cheap
BRS GFO and Carbon Reactor - English
I was thisclose to buying one my self until I got my phosphates down to .13 and I figured i can do the rest myself. The one I linked can also tumble carbon too soif you need that this is a great little reactor.