I've used NoPox to bring nitrates from 160 plus to zero. I dose 4 ml daily to 16 gallons for reference. Since your nitrites are zero they aren't exaggerating your nitrate reading. Key with using API nitrate test kit is properly shaking that second regent. It's not precise but accurate enough. Easy enough to determine when it's below 25 ppm. WWC runs their systems at that level. Panic would be the last thing I'd do. Plus high nitrates shouldn't be the cause of fish deaths. At least not from a sudden rise but more likely from prolong exposure. ICP might be the only route to exclude anything else causing issues.
BTW, since the test only goes to 160 ppm then there's no indication of exactly how high is high. Couple of 25% WC would help although I try carbon dosing first. Vodka more concentrated than vinegar but those two are an option to NoPox and readily available. Often found in most homes. Just wouldn't use the expensive vodka. Perhaps what is given to guests