ammo up

Could have been the chlorine or chloramine in the tap water. Did you treat the water with anything before you added it? That would be my only guess.

I wouldn't say nitrates of 40ppm would effect the fish.
 
I would take a close look at whatever you used to transfer the water in. Mabey something was used in it (soap, cleaner, etc..) that caused a problem. Also have your tap water tested for any type of new chemicals that the water company may have put in. P.S. Sorry for yuor loss
 
I would suspect contamination of the water container or something new in your tap water.

Also it is possible you stirred something up in the substrate.
 
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