This is the most common question in ponding, the easiest to answer, and sometimes can be the hardest to “fix”.  Green water is caused by free-floating algae cells in the water column, as opposed to the types of algae that colonize the bottom and sides of the pond.  All algae growth happens simply because there’s sufficient sunlight and lots of plant food in the water. All pond plants feed on nitrates and phosphates, but algae can reproduce so quickly and exponentially that they out compete with the pond plants creating a green soup.

         How do I keep the water from turning green?
     Removing excess nutrients and minimizing sunlight are keys to help control green soupy water. During the season, algae blooms are usually caused by overfeeding, fertilizer runoff and/or hot summer sun. You’ll want to eliminate these sources of nutrients first.  Only feed your fish what they can completely consume in a few minutes.  Any more is algae food.  Make sure no runoff is getting into your pond, not only to keep fertilizers out but pesticides, fungicides and other toxic chemicals as well.  Grass clippings can be a major culprit if you have to mow around the perimeter of your water feature.  If you still have green water you may not have enough ornamental plants to consume the nitrates that are a natural end product of good biological filtration.  Consider adding pond plants to consume the nitrates and out compete the growth of algae and, if possible, shade the surface of the water with floating plants (such as Lilies, Hyacinths and Water Lettuce) to remove another key component of algae growth – sunlight.  With about 60% of the surface of the water covered with lily pads or floating plants, algae just can’t thrive.

           Should I use a UV Clarifier to control algae?
You can use ultraviolet light to control floating algae, by pumping water through a tube containing a UV light source.  UV light at the appropriate wavelength and exposure actually damages the DNA of algae cells, preventing them from reproducing effectively.  Since individual cells typically only live a few days, they die off by themselves without reproducing, so the water clears. UV is most effective when used for those spring or Indian summer algae blooms when the other plants in the water garden are dormant. Using a UV light/filter is a very effective way of keeping the pond water crystal clear.