Planktonic is a term that covers all forms of algae that are suspended in water.
These algae have thread like tails, called flagella that allow them to move through the water.
This type of algae is a waterborne microscopic plant that reproduces like crazy and becomes so thick that it actually colors the water green.
Not only is the green pond water not a pretty sight to look at put it also puts your fishes health at risk.
Extreme cases of green pond water can cause severe dissolved oxygen depletion, which threatens the life of any fish and invertebrates.
Ways you can keep the water clear and healthy for fish are:

• Do not over feed your fish.
  Feed your fish only what they can eat in 3 to 5 minutes, twice a day.
  If there is any food left after feeding, always remove it.
  If you let organic matter accumulate or offer excessive food, the resulting population explosion of bacteria will lead to an algae break out and green pond water.
  
• Adding oxygenating plants or submerged plants to your pond.
  They act as natural filters and remove excess nutrients from the pond, keeping the water clear.
  The higher the nutrient level in the pond, the more algae you will have.
  
• Adding floating plants to your pond (like water lilies, water hyacinths, and water lettuce) will provide shade.
  Algae need sunlight to survive so any shade you can supply will help keep the algae down.
  Floating plants will also compete with algae for nutrients in the water.
  The more plants you have, the more the algae starve and reproduce less.
  
• Install the proper equipment when you build your pond.
  To keep the water clear and healthy for fish you must have a pump and filter.
  You need to make sure and get the right size for your pond.
  Your pump should move at least the full volume of the pond water each hour.
  The filter needs to run 24 hours a day 7 days a week to work.
  
• Last but not least, this piece of equipment will work wonders on string algae and that is the UV sterilizer.
  When installed correctly, a UV sterilizer will show outstanding results within a matter of a day or two.
  As the water passes through the sterilizer, ultraviolet rays kill algae and other harmful bacteria.
  For a UV sterilizer to be effective you need to purchase one that is equivalent to your pond size.
  To win the war against string algae you must have a UV sterilizer.
  

To prevent algae you need to address the cause, rising levels of nutrients (nitrates and phosphates) arriving through food and even in the tap water.
Excessive food relative to pond volume results in an accumulation of nutrients on which algae thrive, boosted by abundant sunlight in a clean pond.
By following the instructions in this article you will know how to keep the water clear and healthy for fish.