Cyanurus acid (C3H3N3O3, Cyanuric Acid, Cya). Acts as sunscreen for chlorine (prevents its burning with the sun), but reduces the effectiveness of chlorine. The more cyanuroic acid, the more chlorine is required to prevent flowering of algae. Ideal levels of 30-50 for pools without a chlorination generator, 70-80 for pools with a generator, 0-20 for pools in the room.
