HTH (high test hypochlorite) 3 In. super 4-in-1 chlorinating tablets sanitize, protect against the sun, controls algae, and clarifies. For use in feeders. Designed to dissolve slowly. Active ingredients: Trichlors-S-Triazinetrione 93.6%, zinc sulfate monohydrate 3.5%, other ingredients 2.9%, available chlorine 84%. Online purchases are in packages of 2.