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I think most nitrate kits will work on saltwater or fresh water, but I'm not 100% sure. I doubt that any nitrate could be coming from the RO/DI if the TDS meter is accurate, so I suspect your measurement there is correct, even if by accident. There might be some nitrate in the salt mix, but I suspect that the problem is in the tank. Have you tested for nitrite recently? Nitrite will interfere with nitrate test kits, although that problem is very rare.
Even with low nutrient input, a buildup of debris in a coarse substrate, for example, can lead to a higher nitrate level. Better skimming might help, too.
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Jonathan Bertoni
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