Unfortunatly, I live in Denver, CO.. and its quite dry here. Humidity in the single digits to teens frequently throughout the summer. I can do what you suggested -- But I'd be filling up 5 gallons a day of waterI don't have any recommendations for water coolers, but have you tried other ways of cooling the water before you put down the cash for a water cooler? Do you have a cover over your tank? You could take off the cover, this can lower the temp by a few degrees. You could rig some computer case fans over the surface of the water to aid in evaporation.
I'm not.. I just did not want there to be an issue.Are you noticing your plants suffering with the temps in the high 70's or do you have livestock that can't take it? Last summer my 120gal tank got up to almost 90 during the day due to my apartment being 85 or so plus 2 175w MH lights about 3-4 inches off the water. The plants and fish all did just fine. On that note I don't think there are any decent (maybe not any) chillers that will work on a 75 and are under 300. The fans will bring down the temp quite a bit but like you said you will have to replace quite a bit of water.