I used frozen bottles of water for a couple of years and they work in the winter after a while I switched to coco coir, same growth rate and no root rot problems. I have an RDWC with a chiller that works great but it's not the way to go with single bucket grows. Are you using Hydro Guard, that does a lot to protect the roots from high temps. There is another one called Southern Ag that is 99X more concentrated if your water temps are extreme.