I don’t know if this helps but I’ve tried fritz, coral pro, and petco brands. I noticed that ALL had initial nitrate when freshly mixed as well! I contacted the companies and actually learned that it’s normal for there to be some nitrates when freshly mixing salt because the salt is allowed a certain amount of decayed compounds (Think about how good allow a limit on things like bugs or random particulate in the foods you eat like chocolate from factories or flies in a milk factory...If you’ve ever been in one, like mayfield for instance, they’re overrun with flies and have machines in place to zap/capture them)...but some amount have to be accounted for. I use an RODI system...I’m not sure if you can add on a DI resin but it does help make nitrates 0 in the water being made. Also, if you circulate the freshly mixed saltwater, after some hours if you check, the nitrates will go down. I think that’s part of the reason salt companies advise reefers to mix the water for set hours before adding straight into your tank....that and it helps with oxygenation. Hopes this helps! I know the if you call the Petco salt brand, they were super helpful at explaining why in a more scientific manner.