There are a few things I would do to reduce nitrate:
1. 30% water changes every couple of days to get nitrates below 10 ppm. Also, double check that your test kit is reading correctly.
2. Get some macroalgae to slurp up some of these nutrients.
3. If possible, throw on a HOB mechanic filter for a few days. Use a turkey baster to baste the rock and sandbed. This will stir up a lot of nitrate- and phosphate-producing crud that can be exported from your system by cleaning the filter. For best results, plan to do this over a weekend when you can baste the tank, let the crud fill up the filter, clean the filter, then do it over and over again until the tank is barely cloudy after basting. This is more work than a lot of people care to put into their tanks, but well worth the effort in the long run.
4. And do weekly water changes of at least 15% forevermore
