Nitrates don't kill fish instantly, it would take months for fish to die cause nitrates toxic at VERY high level, I did say about my Tesselata saltwater live in 100+ Nitrate environment for months if not year before I research enough how to combat Nitrate with carbon dosing in saltwater. So I would say it's just some random death like we usually seen happen with hydrolycus scomberoides species, morning is perfectly fine and just fall death few hours later.