I have several "reactors" or canister filters I've made form PVC for teh occasions I need to run GAC or GFO temporarily. So I'd say go with a small one with it's own pump with just a few turnovers an hour, it'll be easier to remove and clean. It doesn't really matter where you run it but I'll go for easiest access.
You didn't mention what level your po4 is. FWIW
.03 mg/l is the minimum threshold level to prevent phosphorus limitation in corals, identified by researchers at Southampton University in England using corals maintained in an aquarium for about a decade. .3 mg/l is what corals are subjected to with upwelling in the ocean.
.5 mg/l might be acceptable as acros have been shown to increase growth up to this level.
.9mg/l is the level teh Steinhart Aqaurium's ex situ sexually reproduced acro colonies are being grwon out with. Rich Ross's (who's part of the team at Steinhart) has his acro dominate mixed reef sitting at 1.8 mg/l, his tank can be seen at the beginning of his
MACNA presentation on phosphate.