Scum on Roots/ Res Temperature High

Hello GC,

To bring everyone up to speed, this grow is utilizing bubble buckets (not top drip feed), with Botanicare PBP line. The grow has been using half solution for the first week, and after a 24 hour flush of pure water (ph'd), they have just been placed in about 3/4 strength solution (900ppm or 15mL per gallon/ pH'd to 5.85-5.95).

It was after 18 hours of being in the new solution that I noticed a scum buildup on the roots! The plant tops were showing signs of distress (pictures below), but I was under the impression it was due to the flush and the utilizing of the last nutrients in the rockwool/leaves.

Reasons:

1) The pH had gone up to 6.6-6.7 for all buckets! I just brought them back down to 5.9 but why did they go up so much after only 18 hours?

2) The res temp is at 77.5 . This was also at a decent 70-ish range, but this could be a big problem with bacteria and algae growth i'm assuming (possibly the cause to what I am now suspecting as Root Rot). Out of curiosity, what are optimal res temps to shoot for?

Edit: To counteract the heat in the res, I'm going to try aluminum tape around my black buckets. Black in itself absorbs light and stores it as heat (compared to white or clear colors). The aluminum tape SHOULD reflect most of the light outward, thus not storing it as heat, and hopefully chilling the water back down... right?

How should I proceed forward? I know there are some res cleaners on the market that keep the nutrient water safe for plants but kill bacteria, or that you can add a certain amount of either bleach or pool tablets to have the same affect? Also, for cooling the water, is there a more independent way other than 4-5 ice'd water bottles throughout the day?

Pictures:

1) Roots just after flushing (white, and a tiny bit of scum)
2) Roots AFTER 18 hours of new solution
3) Pictures of foliage are all from AFTER the 18 hours

Thanks for all your help in advance!