lowering tank temp.

My parents live in a trailer in SC, USA with no airconditioning. My dad would take well water, so obviously no clorine, and make ice cubes and put one at a time in the tank if it got to hot. His angelfish would push it around the tank playing with it.

if you just want to cool the water with out adding the cubes directly to the tank you can put them in a zip seal bag with some water to displace the air around the cubes.

If it is a more long term problem you might look at a chiller.