Chaeto is not going to be enough to remove PO3 IMO. Not sure of the ratio (and from what I understand its a best educated guess anyways, no hard facts) but chaeto will remove something like 1 PO3 to 4-6 NO3.
I fought it left and right to only have "natural" filtration. Live rocks, fuge, chaeto and skimmer only. I had to give in. The chaeto could not keep enough PO3 out of my tank. I gave in and bought some GFO and a reactor. Now my PO3 is undetectable and my hair algae disappeared in just a few days once all the water cycled through my reactor.