There are several different types of algae that can develop in a hot tub, and some are easier to get rid of than others. Blue-green algae is one of the most common types; it's the same organism that causes blackish growths on bathroom tiles, sink faucets and in fish tanks. Chlorine, often used as a sanitizing agent in hot tubs, will not kill this type of algae. Green algae is the fastest growing type of algae, so if the water transforms from clear to thick with algae overnight, this is most likely the type. If the sides and bottom of the hot tub begin to feel slimy, this is one of the early warning signs that green algae is present. The other type is mustard algae, a highly resistant form that loves wet, shady conditions and is easily identifiable by its yellow, powdery appearance.
Hot tubs that are covered when not in use, and are equipped with an ozone generator, rarely exhibit algae problems. The appearance of algae may indicate that there is something wrong with the generator, which turns oxygen into ozone and pumps it through the water to kill bacteria and microorganisms. A hot tub with an improper balance of chemicals and pH can encourage the growth of algae, while leaving the cover off for several sunny days may provide the organisms with enough sunlight to start growing.
Algaecides are chemicals formulated to kill algae. Different types of algaecide can be used, depending on the type of algae. Polyquat algaecides are effective in treating all three main types of algae, but for a particularly bad infestation of the resistant varieties another type of treatment might be necessary. For mustard algae, chelated copper can be particularly effective, as can an algaecide containing bromine. Increasing the amount of free chlorine in the hot tub will also help. For every dose of chlorine put into the water, some of it goes toward killing bacteria and other organisms. Testing the water's pH level and making sure it's in the right range--between 7.2 and 7.8--will help discourage algae growth.
Scrubbing the inside of the hot tub will also help. This will free any algae that has attached itself to the sides so it can be filtered out.
Test the spa's water regularly, and keep the pH within the recommended range to discourage algae growth. Whether chlorine or bromine is used, keeping these chemicals at the proper range--between 1.5 and 3 parts per million (ppm) for chlorine and between 3 and 5 ppm for bromine--will help ensure that any algae that begins growing is killed before it can multiply.
Covering the hot tub will also go a long way in preventing the growth of algae. A cover will keep out the sunlight needed for the algae to grow, giving the chemicals time to work.