Yeah I'm in the States.
Your reasons for root rot include the REAL culprit, which is overwatering. Moving the plants is fine for them, don't worry. And I have never used anything BUT untreated tap water!!! Overwatering plus heat= GREAT conditions for mold to grow.
Nextt ime you see that problem strting, try drilling the sides of the buckets fulla holes to let more oxygen in. I have never used hydrogen peroxide, but it IS a possibility to treat root rot. Also, Serenade fungicide used as a drench could help if it is not too far along. If a plant is showing signs of root rot, another thing to do is get it into a spot where it is not getting the full blast of your big lights, because it is not able to suck up moisture fast enough to replace that lost by transpiration.