It's Calcium Hydroxide. In large quantities, it could be toxic for your entire tank. When mixed and added properly, it helps maintain calcium, alkalinity, and even PH. Most people add around a tablespoon per gallon, mix and let settle. The next step is to only use the clear water after it settles and drip it into your tank over time. The clear liquid is very hi in PH and if added to quickly, can raise the PH of the tank to toxic levels.