I had this issue for the longest time and nothing i mean NOTHING was lowering Phosphates and resulting in ridiculous amount of algae till i used Dr. Tim’s Waste away. All my issues of algae, phosphates and nitrates are gone. I of course syphoned out as much algae as possible with each water change. Hope this helps.
what causes this? It’s a new system setup 12/28/19. I’m gonna test my rodi water but the meter shows 0 TDS

