Sudden Betta Fish Death

[super9]

I've had two Bettas die on me within a week of one another in the same manner.

I wake up the morning, feed them and go to work. I come back, they are dead with a blackish patch on their scales with their gills sortha expanded.

They were both living in different tanks. I do however, move them into my planted tank from time to time.

What could possibly kill my fish in 12 hours?

[Guppendler]

Can't really tell. Is the black patch on the stomach area? What did you feed your betta with?

[diazman]

How and what were they housed in?

Care to share WC regime/filtration/parameters etc..

[stressed]

Could it have been the repeated movement that unsettled the fellas?