[Zapins]

I've seen a few different methods. Most involve either using fans blowing across the surface of the water cooling the tank much like blowing on hot soup.

Other methods include using small 40$ fridges and drilling holes in the walls then running a hosepipe into the fridge and keeping the excess hose in a coil sitting in a tub of water.

I am working on using a gutted fridge to directly chill the water (putting the metal freezer box into the tank itself).

 

[Zapins]

Yep, a fan will lower the temperature by a few degrees easily.

If you are looking for larger temperature drops you'll need a heat transfer system (fridge//chiller).

I also tried the ice bottle method but they melted too quickly at 95F!!! They only kept the tank cool for about 2 hours and the saltwater fish quickly got stressed so I had to return them to the sea.