The heat is definitely not helping things, but considering the fact it's spreading I agree with tangent- it's a micronutrient deficiency.
Zero ppm water doesn't have the same micronutrients that filtered tap water does. Also it is highly reactive to pH changes which is why the pH isn't changing very much when you flush them. You will need to flush with some filtered tap water or spring water that is pH'd to 7 instead of the distilled. That will have much more of an effect, and will supply some micronutrients.
I'm thinking you should have added some Peace of Mind or Happy Frog organic ferilizer after about a month if you wanted to stay organic. The soil probably doesn't have enough bacteria breaking it down. Some FoxFarm Big Bloom would have definitely helped too.
I like the idea of using a micronutrient complex at this point like Revive, and some really light Grow Big and Tiger Bloom.
Since Tangent had this type of problem before and fixed it, I would follow his advice
