I'm not a pro but somewhere in the 70s I think would be ideal. I understand how hard that can be to achieve in these climates we live in. CONSISTENCY IS JUST AS IMPORTANT. a hot spiked can make you go downhill quick. So if you are putting Frozen bottles in your Reservoir that means you have to be all over it constantly because one slip-up of a temperature soar and you're in trouble again not to mention that will give you fluctuations that your plant will not like. There are really only two options in my opinion. Either time your grow with the cooler seasons where you can gain complete control. Or invest in some type of thermostatic controlled water cooling system. It sounds extreme but we live in extreme conditions and if you cannot control those Temps it can be disastrous. I really can't emphasize enough how crucial those temperatures are my friend. Hopefully MRmcskunkins will chime in on this he is very knowledgeable with Hydroponics.