I had a DWC for my first grow and I had res temp issues as well. I ended up getting pythium or brown algae and it was a pain in the but. I removed about 3/4 of roots (slough) and treated the plants with Heisenberg Tea. I decided to try using beneficial bacteria to fight the bad stuff. It worked like a charm, but it kept coming back no matter how much I cleaned and kept the res temps cool.
It came back hard and took out one of my plants. She was just kilt over one morning. The others started to droop so I quickly made some cuts and started over.
Nobody wants to hear it, but sometimes the best choice is starting fresh. I'd have been better off scrapping them after the issues first appeared, in retrospect. I wouldn't have learned anything either.
Deep clean your whole setup, anything the water comes into contact with. Keep the temps cool and either go the sterile route with chemicals and such or the microbial route with a compost tea.
I'm not sure if I had pythium or brown algae or both, but I did have brown slime on my roots and hydrogen peroxide didn't help. I used some enzymes one watering and it really went to hell. Worst thing I could've done, it greatly accelerated things and the bad bacteria fed off it and came back twice as strong.
Brown algae, is notorious for returning. I ran a bleach water solution through everything several times and it still got me in the end. I thought I was in the clear after my plants recovered (nicely). But I now know I should've been much much more vigilant in watching for the signs. Starts with a clear slime on the roots or brown crud in the water/sides.
My roots bounced back amazingly, but I used a lot of rooting hormones and such on them in addition to steady applications of Heisenberg Tea. Set me back at least a month, all told.
Here's a link to the Heisenberg Tea thread at rollitup: