I use Fleet Enema to maintain PO4 levels around 0.5-1ppm, you can easily double that amount IF every other element is also well balanced. High PO4 levels can create problems if other imbalances or deficiencies are present.
In absence of a PO4 test kit, I usually find that if I need to dose NO3 to maintain it above 0ppm, I will also need to dose PO4.
Hope that helps
Giancarlo Podio