I’ve listed some below.

1. Believe it or not, the best and often looked over, way to keep an aquarium cool is by moving it away from direct sunlight or away from windows. If it’s not near windows, you could place curtains or blinds on the windows to block the sun’s heat from coming in. This is the easiest and way to lowering aquarium water temperature.
2. If there’s a sudden heat wave or for some reason the sun is just shining brighter than it used to, get a bag of ice from your convenient store and dump it in your fish tank. As you might have guessed this is but a temporary solution, but albeit an effective one.
3. Good ‘ol electric fan. Every house should have one of these in the summer. Sometimes you don’t need air conditioning and just the wind in your face is enough to relax you. The same goes for you aquarium. Sometimes the heat isn’t bad enough to warrant the use of air conditioners or chillers. Sometimes you could get away with just using an electric fan pointed towards your fish tank.
4. If you don’t have an electric fan or want to do any of the above, short of getting an aquarium chiller, you could also use your room’s air conditioning unit instead. Turn it on so it cools the room and eventually your fish tank. This however will reflect on your next electricity bill and should be used as a last option.