Ok.. im guessing you joined the AA for help and information on fish keeping?
Did you see the questions i asked, have you cycled either of these tanks and how much and how often are you doing water changes? You responded to my statement but didnt answer the questions.
I ask because you mention that youre having trouble keeping anything else alive which leads me to believe your tank isn't cycled. Putting fish into a tank that isnt cycled is going to cause death without regular water changes.
Fish die belly upward and rise to the surface. It's their way of falling.