Since it has been pretty hot in CA, my turtle tank temp is about 80-81 degrees during the day. I've tried using frozen water bottles, but according to the calculations from this site, I would need 16 lbs of ice to lower the temp 5 deg for 70 gallons of water.
Heres the link: http://slate.msn.com/id/1003374/
And for the lazy:
1) Take your pool/tank volume, in gallons.
2) Divide by 1,000.
3) Multiply by the number of degrees (Fahrenheit) you'd like to cool the water.
4) Multiply that number by 43.75.



