Who Gives A Fish Forums

How often are you changing the water? And what's the tank size? Could be overfeeding or too much light.

I'm with flame9x on PhosGuard. It's chill and doesn't crash your levels like GFO can. Also, check your RODI water. If your membrane's shot, you might be adding phosphates with top-offs. Happened to me last year—took forever to figure out why my levels wouldn't drop.