Ice packs to chill tank in summer?

[GoFish]

I keep coming home from days on the water to find my temperature climb, so far my display tank has been as high as 84. Lost my Setosa but no big deal. Started throwing ice cubes in the tank to cool it off but that affects salinity and I don't use filtered water to make ice cubes, so instead I started using those plastic sealed ice packs used for coolers, take out of freezer, turn off ATO, drop ice pack in , wait for it to melt completely, remove ice pack turn ATO back on, rinse ice pack, put back in freezer, repeat... It works

Yes I could use a chiller, or make a DiY chiller but I like the simplicity of using ice packs, the freezer is running anyway right. Sounds pretty economical.

I just started freezing a large ziploc with 4 cups of water and will be putting that in a tank volume of 45 gallons. I use a Digital Aquatics controller so it should give a pretty accurate tank temp change before and after. Will post results later on.

Anyone else do this? And is there any calculations that someone knows of in figuring out how many cups of water worth of ice will chill a certain number of gallons by a certain number of degrees.