I would first double check your test kit with a completely different brand of test kit. Some test kits are good for high range some are good for low range and some are just crap or from a bad batch.
I would check the phosphate as well (use the hanna ULR for this). Sometimes when the nitrate is high the phosphate is extremely low. More importantly than high nitrates is you have to have the nitrate phosphates in balance. If low phosphates is the problem it can cause serious problems with your corals. You may be inadvertently adding nitrates to your tank which have crashed your phosphates. What additives are you putting in your tanks?
I crashed my phosphates by adding nitrates last year and within a couple days fried about half of my sps.
If you nitrates are high and your phosphates are equally high you need to ask yourself what have you done (husbandry) to get your nitrates that high? What are you feeding?
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