What's your bioload? Would you say the tank is 'full' of fish? Anything recently dead or dying?
Try even more water changes. You will inevitably triumph this way, but it will require more than calculations suggest.
By far the best way imo is rapidly growing algae in a sump refugium or algae scrubber. In the first tank as pictured in my sig, I have a rather small amount of chaeto in the sump, but under fairly aggressive lighting, and it grows like crazy. Although the tank is crowded and I feed liberally (4x/day), today its nitrate was 0.30 ppm per the new Hanna nitrate tester (and almost undetectable by Red Sea.) As long as the algae grows, I literally cannot raise nitrate through feeding. Although I 'normally' do a small water change once a week, I've been skipping some of those more recently, with nitrate and phosphate levels so low. This is basically like the Triton method, although I don't call it that and don't use their products or follow their rules. Note that if the algae starts to die off, which can happen with chaeto, the nitrate level will start to rise immediately (especially if you fail to remove the chaeto before it decomposes.)
Another way is denitrification. This requires high-surface area blocks or rings, which then sit in a relatively low flow area. In the very deepest parts of these structures, anaerobic bacteria will convert nitrate to free nitrogen gas. That's the theory, anyway. I've never seen conclusive proof that it works, when I've tried it. Unlike an algae fuge, which absolutely works, dramatically so.
Also agree with checking the water you use to make salt. If it's more than 0 ppm TDS, some nitrate could be getting through. Although very unlikely to result in a nitrate level in the 50 ppm range.
