I agree that Rtn is something else. Zero po4 will result in issues. Such as pale sps and pinching lps. But what I see is a result of parameters, alk, or contaminates. Don't take my advise, but research it. If I'm you I'm making sure my RO water is 100% pure, I'm using a dosing pump to keep my numbers solid, I'm leaving the tank alone, dosing P To keep it detectable and stable, (n is fine) and keeping my alk at 7.0 day and night. If that doesn't work I'm letting the tank mature more, and repeat. My .02 cents
