Had the same issue back in May (running about 75ppm). I ran a korallin 1502 for a month (already seeded) on a 220g and wasn't getting anywhere fast (knocked it down to about 25-50). Added another and got my nitrates down to 2.5 and just recently dropped down to one denitrator. One thing that I've noticed is they tend to lower ph a good amount. I'm running a co2 scrubber on a fresh air intake hooked to my skimmer and max out around 8.15 during the day. Fresh air intake alone was dropping down to 7.5 at night with two 1502s running.
Tried other methods such as chemical, but found things like nopox are more suited towards po4 and will stop working once po4 gets low. While the denitrator works well for no3, keep an eye on po4 as mine shot up after I got my no3 down and bleached a couple frags.
They do take a little while to establish so be prepared for a couple more months of high nitrates, but once you're getting zero no3 output you're well on your way (one drop at a time). The other thing to consider is an uplift pump to feed it and something to prevent air bubbles from getting inside. Once the water is flowing, you may not need the uplift pump.