is this a new tank? Low no3 high po4 is a classic sign of immature tank. Early tank biome is more N absorbing (coral, algae) than P absorbing (bacteria, pod, microfauna). Keep up with your water change and manual husbandry will help and over time the P absorbers will stablish.
If you are not already dealing with one, you may have a cyano bloom soon from the high P.
Try to deep vac the sand one small section per WC to help remove the sand waste nutrient battery.
If you are feeding "high p" coral foods, try switching to a high N low P source like phyto or aminos