The safest way to treat white spot is to raise the water temperature to 30C (86F) and keep it there for 2 weeks.
Before raising the temperature, do the following:
Wipe the inside of the glass down with a clean fish sponge.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
Do a 75-90% water change and gravel clean the substrate before raising the temperature. This will reduce any parasites in the water and gravel.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Increase aeration/ surface turbulence to maximise oxygen levels in the water.
Have a coverglass on the tank and insulate the base, back and sides with polystyrene foam so the heater doesn't have to work as hard. This is more of an issue in cold rooms.