Had a bump in PO4 for the past two weeks, been monitoring but not coming down. I auto-water change 10% / 25 gallons a week split over the seven days and have approx. 30 gallons mixed each Sunday for the coming week using RODI and Tropic Marin Pro salt - Have been doing this for a couple of years without issue. Just PO4 tested the fresh salt mix with my Hannah Phos URL - and got 0.07ppm! I articles with people reporting similar issues with Red Sea salt but no real explanation. I've not tested my RODI as I don't believe the Hanna will work on freshwater? Anyone else had a similar issue and found a cause?