Heat kills deadly

[Delphinus]

Are you in an apartment or something?

What are your nighttime temps?

I'm a little perplexed that you've lost fish. 90 shouldn't be killing fish, stressing maybe, but killing ... ? You say your heaters are off so I wonder what kind of swings your tanks are encountering. Or maybe it's some kind of factor of the speed of the temperature spike.

I think that lowering the setpoint on your heater would be a better option than taking it out or leaving it off. You need some sort of stability in your temperature management. Temperatures do cool off at night, and IMO the swings and the rate of change can be worse than the actual peak values.

As far as weathering the heat, ... evaporation is your friend. Increase it as much as you can. Add fans, remove covers. Potentially adjust your photoperiod so that it does not coincide with the daytime peaks.

Also, a fan blowing ONTO the water surface will do more than just a fan blowing ACROSS the water surface, so keep your options open and be creative.

Or if you've got the cash, get a chiller or a room AC unit.