Terribly sorry to hear about your fish kill!

I would say that you have brought too much water up from the bottom, too fast, that was void of oxygen. This bottom water mixed with the upper levels of water WITH oxygen. This would reduce the overall oxygen levels and cause the fish to suffocate or nearly suffocate.

This water movement that occurred also brought up gasses that typically collect and stay near the bottom. These gasses can be poisonous.

I'm no expert and hopefully you will get a more specific explanation to the cause, but at this point you need to know how to move forward without making things worse. Is your aerator running now? I would suggest that it stays off until your situation has been thoroughly evaluated.


Fish on!,
Noel