Here's an idea I read in a post a while ago for cooling the water in your tank if you don't wanna buy chillers (not sure if it's already posted here or not).
Connect a long hose to the output of a pump inside your tank, run the hose inside a bucket full of ice, and stick the other end of the hose back in the tank. This way the tank water goes through the hose, inside the bucket of ice, returning cool water back into the tank.
You can change the level of cooling by adjusting the length of the hose that's sitting in the ice bucket.
If you have a small fridge or freezer you can run the hose inside the fridge, so you don't have to buy blocks of ice (not sure if it would be cheaper though).