I thought colder water makes it so that more air can get in the water and thats one reason DWC use chillers the lower the temp the more air it can hold .
High reservoir temperatures provide an ideal environment for bad bacteria like pythium to thrive. Mid 60s is the sweet spot in terms of Dissolved Oxygen, as well as bring a comfortable temp for roots to thrive (And keep the baddies dormant). I believe dissolved oxygen levels with respect to temperature is a bell curve, with inadequate amounts of oxygen being present on both of the extreme ends of the spectrum.
My last grow took place in an unheated basement. All I had was my dehumidifier and the lights to regulate my temps. The girls.never saw a day over 65. With an inoculated medium, loads of silica, and foliage feeding I was able to keep them thriving but once you get into the low 50s the plants really start to shut down on the metabolic level.
Anyway, I definitely wouldn't advise ice water. Stick your hand in a bucket of ice water, and think about how long it will take for that water to warm up to a temperature that won't be painful. That's how long your plants most sensitive and important area will be exposed to those conditions. You probably won't kill it, but I can't imagine it NOT stressing the plant out significantly.
A good rule of thumb is the environment causes discomfort to you, it won't be good for your plants. Whether it be intense heat from close proximity lighting, or freezing water sitting in a pot with your root system.
Also, like Dab said, you are at the very least a few weeks away. She still has quite a bit of growth to go before she will be ready
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