Oh sorry I know my post was quite confusing.Your photos accompanied by your text are confusing to me. But if I take your last photo as the "true" one with nitrate, I don't see the problem. Nitrate looks like it's around 40, nitrite around 2? Ammonia still high.
Does it matter that you're using a freshwater kit?
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Your LFS was sort of right. Reagents are the same, however, the color charts are different for freshwater vs saltwater. I can take a picture of the cards when I get home to show you the differences. I believe you can contact API and get saltwater cards. API is fine for your cycle, but they aren't great test kits. I'd look to get something else once you're up and running (Hanna, Salifert, Red Sea).
Having Nitrites will affect the nitrate test. That’s why you saw high nitrates then low nitrates when your nitrites dropped.