When treating my water for a WC I:
1) Fill up my 60L container from tap
2) Heat to 29C (84F)
3) Run an air pump
4) Add 1 teaspoon of sodium biphosphate (ph Down)
5) Put lid on, leave water for 24hrs then do the water change
Letting the water stand for 24hrs and running an air pump through gets rid of any chlorine and other unwanted chemicals. It also gives me time to adjust my temperature and pH. This way I dont need to keep spending money on buying instant chlorine neutralisation chemicals, and I can get my water to the right parameters at the same time.
Hope this helps, but I only do this because I keep discus (very sensitive to water quality)
In regards to white spot, I've never had to treat it so I dont know sorry. But I think raising the temperature may hinder the bacteria propagation somewhat, but you should consult an expert or your LFS on what to do.
Good luck
