Pool Frog PAC 540C provides pool owners who utilize the Frog XL Pro Chlorinator with a simple, drop-in chlorine insert that allows the XL Pro system to slowly and steadily release needed amount of chlorine for maximum effectiveness. PAC 540C gives users a clean alternative to handling messy chlorine tablets or liquids, and when used with the XL Pro system, is completely automated to remove adding chlorine on a daily basis from your busy schedule.