A few weeks ago I put a mouse pad in my sump underneath my return pump, hoping the rubber would make it less noisy. Thought nothing of it.
Two days later I had fish dying in droves.
Hubby soon realized it was the mouse pad which was actually a two-layer mouse pad with a layer of glue in between which was dissolving into the water.
Many frantic water changes and the deaths abruptly stopped, but my Angel, who lives, is brain damaged (no joke here, and it's sad)
So I poisoned my fish
And the one who lived will never be the same.