High phosfates??

So i have a 3 month old tank. I had the normal diatom outbreak bc I started my 32 gallon with tap water. After the nitrogen cycle I started doing weekly 5 gallon water changes using rodi water and instant ocean salt and the algae problem went away for a while until I got a cyano on my sand bed. I added some purigen and phosguard and knocked the cyano out ,oh and with the help of a diamond goby.

I got a Salifert po4 test kit and checked my water last night and came up with a reading of .5, which isn't good from what I read. (Note i checked my tap water and my rodi saltwater and they had a zero po4 reading) my question is how bad is high po4? And what should i do to get rid of it and where is it coming from. Should i use gfo? Also when I added the phosguard my corals shrunk so I only left it in for 12 hours..

Stock- fire fish, clown, 1 turbo snail , 5 small snails, diamond goby, peppermint shrimp. Flower pot coral, mushroom, rbta,.
Feed every other day freed dried myisis. Very small amounts too.

Tank
S.g. 1.026
Ammonia and nitrites 0
Nitrates sub 5
calcium is low at 325
mag was 1200
Alk. 8.4
Ph 8
Switching to a reef salt to try and boost some of those levels next week.

Any help would be appreciated..