We use these in a 2-3 person spa/hot tub. They are very convenient to use, just pop one in and thats it. It floats on the surface and is colored blue on the top, silver on the bottom. When it's out of Chlorine it rolls over and shows the silver side, time to put a new one in. Very easy. They last about 4-5 months in our spa. They maintain the chlorine level to the correct level, we rarely need to add any additional except the occasional shock treatment every few weeks. Walmart had the cheapest price with delivery so thats who we used!