You can run lights normally in a QT with CP. Its the powder itself that is light/heat sensitive.
However, if you are using CP (or copper) to treat fish with velvet I would keep the lights off as much as possible. Velvet causes fish to become overly sensitive to light.
If a chemical is light sensitive in substance, it is usually even more light sensitive in solution. So i would use only dim lighting when medicating with CP.
Best,
Christoph