you can make a homemade tank chiller pretty easily. all you need is a small fridge (the kind kids get for college dorms), about 100-200 feet of flexible tubing and one of those heavy duty out of tank water pumps. The premise is pretty simple, drill two holes in the fridge big enough for the tubing to pass through and then just coil as much tubing as you possibly can into the fridge. As the water passes through the fridge, it cools down (why you need about 100 feet of tubing in the fridge, so the water dosnt pass to quickly).
Now i have never made one of these, i have just read about them being made on saltwater forums. It seems like it would be pretty easy to make and work really well though, but an airconditioner would probally be cheaper and less time consuming.