5kg Castle Hot Tubs Small Chlorine Tablets (20g) for use in swimming pools and hot tubs, water treatment chemicals for spas. This quality 20g Small Chlorine tablet is designed for use with chlorine feeders or with floating dispensers. This tablet slowly erodes over a period of 3-5 days depending on water flow. It is high in Chlorine content. Prior to using this product adjust the pH levels to the ideal range of 7.2-7.6 and as this product releases chlorine relatively slowly it may be necessary to shock dose or oxidise the pool or hot tub prior to its first use.