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A heavy metal chance contaminant would likely cause a slow degradation in the tank as opposed to a sudden massive die off, this would include a small mass copper contaminant. A bacterial bloom causing hypoxia would leave its telltale cloudy water and slimy residue, as well as killing the more biologically active fish. It really sounds like some sort of poisoning affecting the invertebrates but fish sparing. What about some sort of pesticide, insecticide, that came in with the frozen food as a collection area contaminant?
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