Green water is caused by an imbalance in the chemistry of the pond.
It is not generally harmful to anything, but it is unsightly. Refer to our Keys To Clear Water page for in-depth advice. Stringy algae can occur even in ponds with clear water. It usually grows on waterfalls or close to the surface, and can only be removed by pulling it off by hand. A good rule of thumb is one inch of fish for each ten gallons of water.