I have used NoPox daily for ~2 years now, my NO3 sits <1 and my PO4 ~.08. I have seen significantly reduced algae growth, but it isn't gone completely - snails fix some of the left overs.
Things I have noticed:
-Constant dosing at too high of a level (for example not following the dosing recommendation based on NO3 levels) will cause bacteria blooms - making the tank cloudy
-Dosing and not testing (or forgetting to test) can lead to undetectable NO3 and kill corals
-Immediately upon dosing pH will briefly spike down until it has fully diluted (if using pH probe - don't know about standard pH tests)
Recommendations:
-Test NO3 2x/week
-If running NO3 below 2, strongly recommend dosing amino acids/carbohydrates/vitamins (I personally use Reef Energy)
-I noticed a lack of coloration once my NO3 were consistently below 2, rather than raise NO3 I decided to maintain them and add RE to keep algaes at bay, it also led to a dramatic increase in color, better than when NO3 were higher
-Make sure you have a good, consistent skimmer
-If you have a skimmer that likes to stop, stop dosing NoPox until you have a better/repaired one