i use mostly peat and when i water i been going with straight tap water phed to 6.0 and have no signs of deffencies like yours are showing and my growroom get closer to 90 on a regular basis is why i say its not from the heat unless you just put that fan in there next time you water your plant check the run-off water aswell see how much of a diferance there is in whats going in and whats comeing out if you got a tds pen you can evan check that or evan a ec meter but i'm still saying it's more then likely a nute lock-up cuased by improper ph for the substraight your useing