I ran into a similar issue with silica. My DI cartridge wasn't functioning correctly and I kept getting diatoms after water changes. I stopped assuming my DI cartridge was OK and got a TDS meter to actually measure it and saw the DI cartridge was bad. Painful mistake that could have been avoided by a ten dollar purchaseFor what it's worth, I was struggling with the same thing and could not for the life of me figure out where it was coming from. Turns out the TDS meter reading 0 on my RODI was a lie. Every water change just added more PO4. Every time the ATO fired off, more PO4 added. Nothing more than a PSA, but it's worth checking on your end.