I think you may have answered your own question when you said the fish looked like they were gasping for air, they probably were, years ago I had a similar crash and lost 22 Koi angelfish, all my water parameters were perfect, fish were all plump and happy the night before, sometimes when you go and service the tank when you refill after stirring everything up, it depletes the oxygen and they litterally suffocate. I had been doing fw tanks for years and never had a problem, until that happened. Took a while to figure it out as well.
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