Then the best I can suggest would be to buy a JBJ Arctica chiller. They are the best for the money. I have used one for years.
Chillers are a lot of money; but what isn't in this hobby?
You can spend a few hundred on a DIY chiller that won't work too well or just save that money and buy a nice chiller.
You could crank up the air conditioning in the room and use a fan over the tank in the meantime.