How long has this tank been set up? Often new tanks will have bubbles on top of the water, this will go away...
The thing that concerns me is your statement about "when you turn on the power filters"....You should keep your filters on at all times, this is very important as the bacteria in the filter needs constant air and food to stay healthy, reproduce, and be doing thier job of accomplishing the nitrogen cycle. If you are shutting it off, the ammonia will be building up in your tank. Thus fish dieing...
You do need to be testing the water for ammonia, nitrite and nitrate.
That is the only way you will know where you are in the cycle.
So post those readings please, and in the mean time I would suggest doing several 25-50 % water changes just to improve the water quality. We will be able to give more advice with those readings.
