Gnats munch on composted materials, not your plants. Doubtful it's bugs, or more the leaves would show signs, and you would have mentioned the sighting. (you've looked for bugs, right?)

Heat stress generally starts at the top, working it's way down.

You've been givin the benefit of my knowledge, but since you seem to discard opinion...good luck. :jointsmile:

But if you desire...you might want to fill-out the troubleshooting form, and hope someone comes along with another explination more to your liking. :thumbsup: