Mold in a hot tub is not only unattractive and smelly, but it can be a serious health hazard as well. Infested hot tubs must be treated aggressively and immediately, but the best strategy is to prevent the mould from getting started in the hot tub in the first place.
White Mold and Pink Slime
White mould, a fungi, is the most likely cause of mould problems in a hot tub. White mould is covered with a protective surface called biofilm. The biofilm, which looks like white tissue paper or toilet paper, is resistant to chlorine, bromine and biguanide. Because they are resistant to the chemicals that usually keep a hot tub clean, white mould and pink slime should be prevented whenever possible.
Prevention depends primarily on observing the highest standards of hot tub maintenance. Those standards include brushing and cleaning all hot tub surfaces once a week. Don't forget steps, jet recesses, pillows, toys, floats and solar blankets. Expose the hot tub to direct sunlight whenever possible. Remove the lid from the skimmer to allow sunlight to reach it, but be extremely careful that no one falls in. Add oxidising chemicals into the skimmer to clean any biofilm out of the filtration lines. Allow the water from the garden hose to run 2 or 3 minutes before adding water to the hot tub. Clean the fool filter every 4 to 6 weeks with chemical cleaners. Add a maintenance amount of shock every week. Shocking a hot tub involves adding a sufficient amount of chlorine (sodium dichlor) or non-chlorine shock (potassium monopersulfate or MPS) to break down organic waste contaminants in the spa. The amount of shock you need to add depends on the size of the hot tub. Run the pool filter at least 6 hours a day. Clean and rinse any brushes, hoses and vacuums used to clean the hot tub. Make sure the water's pH, total alkalinity and calcium hardness are correct. The pH for a hot tub should be between 7.2 and 7.6; total alkalinity should be 80 to 120 parts per million; calcium hardness should be 150 to 300 parts per million.
If your hot tub already has white mould or pink slime, treat it immediately and thoroughly. Clean all surfaces to remove all visible white mould and pink slime. Clean the inside of the filter cartridge and the spa equipment pack. Shock the hot tub with three or four times the normal dose of shock treatment. Drain, flush and refill the hot tub. Test the water and balance it. If you can, check under the hot tub for leaks. If you have a leak, the area underneath the hot tub may be infested with mould, as well, and will require thorough cleaning.
Mold exposure can cause a raft of health problems, ranging from skin irritation to difficulty breathing. After you have inspected and treated your hot tub for white mould and pink slime, test the air beneath and around the tub for mould. If the test indicates mould, find it and treat it.