If you're considering TA's Nitrate Destroyer check out Vodka dosing, which is essentially the same idea - provide a carbon source.
I would also up the water changes. I have found that 20% every 2 weeks works better than 10% every week, but that's just my tank. Try playing around with that a bit, and see what you can achieve. People under estimate the Power Of The Water Change!!
I wouldn't be surprised if your softies are affecting the SPS...your leather is quite large. Mixed reefs are much more difficult than dedicated tanks.