I have a Hannah Checker Phosphate Colorimeter you could borrow if you like. I have never had it tested for accuracy but reads 0 for my tests. I run BRS carbon and gfo mixed in a single reactor. Stirring up crud in your sump might have polluted your water column. The poly filter seems interesting, although I have no experience. You could also have some of my premixed carbon/gfo if you want to give it a shot. Seachem Sea Gel is a similar product I've used that works with a single reactor.
I'm also using rscp from the group buy and have to dose to keep my mg, kh and ca up. Changing salts seems a little dramatic without confirming it is your problem. I'd recommend testing some freshly mixed water. Good idea on mixing the salt but if I were you I'd mix and test the next batch before mixing the salt. That way if that is your problem you'll know. You could then dump that batch, mix the salt, make another batch and retest.
I'm not able to head up that way today due to my wife having to register my daughter for kindergarten when I get off work but you could swing by here or I could bring it up for you later in the week or this weekend.