PoolRx For Spa Unit 400-1000 Gallons 3oz P/N: 101055 There are two parts to this product Minerals in the water Metallic rod Minerals in the water We have packed 2 oz of the minerals into the middle of the rod. This is enough minerals for a 1000gallons / 4,000 liters. If your Spa is larger than that see chart below to work out how many additional SpaRX units you will need. Metallic Rod The rod in the blue cage is a specially treated alloy that has rejuvenating abilities on the copper, silver and zinc minerals. Allowing these minerals to continually defend and attack algae and bacteria in the spa 24hrs a day, 7 days a week and lengthening the replacement time to 4 to 6 months. Yes thats right! The SpaRX unit only need to be replaced every 4 to 6 months. Where do I install it? Simply Place the SpaRX blue cage unit into the skimmer or between the filter cartridges. Where ever it gets the best water flow over it. What happens when I place the unit in the skimmer or between the cartridge filters? The blue caged SpaRX unit should be placed in the skimmer at the beginning of the filter cycle. This will give the minerals time to dissolve and get through the filter into the spa water where the minerals need to be. The minerals go out into the whole volume of water and form a stable residual, allowing the minerals to eliminate algae and bacteria where ever they are in the water. These minerals are totally effective against algae and bacteria but totally harmless to bathers. How often do I replace the unit? The unit needs to be replaced every year every 4 to 6 months. If not, the treated metallic rod over the 6 months gets oxidized and calcified so when the minerals pass back over it they do not get the ion exchange between the minerals and the rod. Why such a big variance, 4 to 6 months? Every spa is different. Some spas need more sanitizing than others. High bather load, leaves and debris, dogs swimming, algae bloom and bad circulation all contribute to how hard the minerals work.