Nitrate will not go down...

[Carandiru]

So I have been battling higher nitrates for some time now. I reduced my feeding. I bought a biopellet reactor (aquamaxx biomaxx reactor). I run rowaphos aswell. Still the nitrates haven't gone down. I vacummed my sand bed and pulled out a lot of brown stuff. The sandbed is only 1 to 2 inches deep max. There is a lot of live rock, 90 gallons more than 90 pounds. The tank is about a year old now. The phosphates are no going lower than 0.29. This is measured with the hanna meter. The Seachem test consistently shows nitrate as a deep purple which is around 40 to 50.
My alkalinity is 8 to 8.9 dKh. PH is consistent from 8.1 to 8.4. 0 ammonia 0 nitrite.

I've been trying out prodibio for over a month and keeping up with the regular dosing. I've also been dosing AZNO3 for a couple weeks now with no side affects except notcing my skimmer working more efficiently pulling out lots of crap. Also noticed brown fuzzy slimmy stuff (I think it's biofilm ) growing on the sand bed after dosing this AZNO3.

No hair algae outbreaks, that stopped quite some time ago.

So what am I doing wrong, my goal is to keep SPS corals. My current soft / LPS corals are doing good.

Any ideas?