When mold grows in pool water, it usually floats on the surface and looks a little like tissue paper. It's frequently accompanied by a pinkish residue known as pink slime, which is a bacteria. Neither of these non-algae growths are dangerous to humans, but neither is desirable, either. Their presence usually means a problem with the filtration system or chlorine levels. White water mold is highly resistant to halogen-based sanitizers, such as chlorine and bromine, but you can still eradicate by it triple- or quadruple-shocking the water. Keeping it at bay calls for meticulous cleaning, because even a tiny amount can quickly grow into a full-sized mold colony.
What Is Water Mold?
Scientists used to classify Saprolegnia spp. -- of which white water mold is an example -- as a fungus, but research has shown the class of organisms to which they belong, Oomycota, differs from fungi in significant ways. Oomycota feed on organic matter and can injure or kill fish and other marine animals. The danger to swimmers is minimal, because the organisms die as soon as you get out of the water and dry off. White water mold should be of concern to anyone who sees it growing on their koi pond, however, because the fish could be at risk of infection.
Although you don't have to be concerned about getting sick from water mold, it isn't something you want in your pool. Unfortunately, it's difficult to kill, because its chitin-enclosed cell walls protect it from pool sanitizers. The cell walls aren't completely impervious, though, and very high levels of chlorine can kill the cells. A strategy for exterminating it involves physically removing as much as you can and shocking the pool to take care of the rest.
How to Eradicate an Existing Colony
The presence of mold in the water could indicate problems with the filtration system, so testing the operation of the filter pump is an important prelude to getting rid of the mold. Once you're sure the pump is operating normally, you're ready to start.
Skim as much of the mold as you can from the surface of the water, and then scrub down the sides and bottom of the pool, as well as any components that are permanently submerged. Remove the skimmer basket and let it dry out in the sun, and clean around the jets and behind the ladders.
Triple- or quadruple-shock your pool by adding 3 or 4 pounds of calcium hypochlorite shock per 10,000 gallons. Before adding shock, be sure the pH is between 7.2 and 7.6, or the shock will be ineffective. Add chemicals to adjust the pH, if necessary.
Run the filtration system for at least 24 hours. After this period, backwash the filter or -- if you have a cartridge filter -- remove the cartridge and clean it manually. It's a good idea to use a commercial filter cleaning chemical to kill any organisms lodged in the filter fibers.
Continue to clean the pool and run the filtration system for another four days. After this period, take a sample of the water to a pool dealer for testing. Shock the pool again if the water tests positive for mold.
Vacuum the bottom of the pool to Waste to remove any residue that may have settled. Test the pH and alkalinity again, and add chemicals to bring them to normal levels -- if necessary -- before using the pool.