Odd fish death

[argan]

IMHO That sounds like the reason to me, the paly fragging. According to everything I have read, toxins are tricky because they might not kill a fish right away.

But if they have long term exposure, or multiple fraggings happen, the toxins can build up and the fish cannot get rid of the toxins.

So the toxins just build up and with enough in their bodies, their organs would shut down. little fishy heart attacks.

Where were these fish ranked in terms of Size of all your fish, and how long had they been in your tanks?

JMO