Contains naturally occurring bacteria to clear green water algae and slime, also contains mosquito repellent. One block treats up to 1000L of pond water, lasting up to 3 months. Use as a preventative before algae initially appears or immediately after cleaning an algae infestation. As the block dissolves it realeases naturally occurring bacteria which assist with starving the algae of nutrients to prevent algae growth. Does not harm fish, animals or plants when used as directed. To increase efficiency, position near the filter or pump. Replace once dissolved.