Friend,

A little detective work here.

First be descriptive.. tell us in a detail manner what you do before they start dying.

Second, think carefully what "extra" that you do before they start dying like " water change" or "new food" or ...

Thirdly try your best to descripe how they die. Bloated? Nervous swimming? Dashing around? Breathing heavily?

Finally a picture speak a thousand words. So take a photo....