so you mentioned heat.. what is your reservoir temperature? anything above 70 is prime to develop root rot, which can cause what you're seeing. .it also looks like nutrient burn.
are you roots white and healthy? is there any slime on your roots?
what PPM/ec are you running? what about your ph?
ive had best results running water only for the last 7 days before harvest. makes the final product much smoother.
for your first run in DWC its not bad. dwc is a hard beast to master.. i fked up more than a few rounds dialing it up. and even when you're dialed in, shit can still happen and down your crop in an instant.. its not forgiving at all.