because i had the same problem i tried to do a lot of research, even though i had dealt with it a lot in the past. it seemed this time it was more relentless and started much earlier than i thought it would. first off, yes RO water is number 1. i am finally getting my own system in two weeks but i have been bumming water from my boy for a while now, and he has a TDS meter and just changed his media, so i know it is actually clean. also, if your phosphate is actually "5", that's extremely high. if you mean .5, or even .05, that's a diff story. even .5 is very high though, but the main thing i found out is that hair algae will "eat" your phosphates, therefore giving you a false low reading.
i tested mine and i think it was .06 or .08 using a Milwaukee digital meter, and i was stunned. i had hair algae all over the place, and my phosphates were lower than my friend's tank that has been setup for a long time and maybe has 1/1000 of the hair algae i have?!? something wasn't right. so i read up on it and found out that the algae will suck up phosphates and i think even nitrates, giving you a low reading.
try to prune as much as you can manually, especially right before or during a water change, trying to take out as much as you can from the tank. do not let it get out of control like i did, cuz it WILL take over everything. and if you can fit and afford a phosphate reactor, get one. i haven't hooked one up yet, more because of space than cost (two little fishies sells one for around 40-50 bucks), but it will help greatly. and again, like condor mentioned, PLEASE use RO water. it will change everything. tap water is a reef tank's worst enemy. i cannot stress that enough. hope this helps a little! and good luck, because i know how friggin annoying it can be
