Algaecide; Finally, NOW for that stubborn algae that just will not go away! A simple to use liquid algaecide. Excellent for treating string algae in water garden pools. Not harmful to plants or fish (including Koi)! 1 pint treats 5760 gallons of water, 1 quart treats 11,356 gallons of water. 1/2 gallon treats 23,040 gallons of water.