Without knowing everything about their husbandry, including tank size, substrate, frequency of cleaning, water source, nutrition, etc, or without a physical description of them before the died, even speculating won't do any good.
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...the oldest task in human history: to live on a piece of land without spoiling it."
Aldo Leopold (1938)
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)