I'm just using a domestic canister water filter (Mitsubishi Chemical brand) and my PO4 is almost undetectable (using new Salifert). I guess your high PO4 problem is definitely not caused by your water source but your feeding habit and your system doesn't have sufficient capability to remove PO4. For most of us, PO4 removal medium such as Rowaphos & etc. should always be part of the filtration system. Not something to add in when high level is detected. By then, the accumulation is already way too much for any medium to remove efficiently.
For your case, perhaps you wanna consider a more drastic approach. There are bottled solution out there that can help reduce PO4 significantly... Phosphatbuster or something like that... but that is if you really wanna get rid of PO4 fast.
A 24/7 running FR is a good investment... regardless what water you use