Biguanides

Commonly known as Baquacil or SoftSwim, these are incompatible with any chlorine or bromine product. It is very important to use only those products approved for use with biguanides; otherwise, your pool will turn ugly brown. Usually part of a 3-part system (the sanitizer, a hydrogen peroxide shock, and a preventative algaecide). It works well for people that have reactions to chlorine, but it is quite a bit more expensive. Additional problems are water mold (a growth that looks like tissue paper in the water) or pink slime, a fungus that develops immunity to the sanitizer. There are also formulations available for stand-alone spas. If your spa has ozone, test the levels more often as it may adversely affect the biguanide levels.