That's a decent bio-load but I think your flake might be contributing to excessive po4
more then you know. I use flake about once a week very sparingly since most isn't devoured immediately like the frozen offerings I feed every other day.
IME almost as much flake that gets eaten just falls to the bottom and is not eagerly eaten by the clean up crew.
Seaweed (which I assume you feed your tangs) can also add po4
if any breaks from the clip and falls behind the rocks.
Frozen if thawed and rinsed before adding to the tank adds very little po4
to the tank.