Coping With Algae

Green water is caused by the growth of thousands of single-celled plants in the water. Algae are quite natural – their spores are present in the air and will colonize any new body of water. There are numerous different types of algae and they all thrive in sunny warm conditions and in nutrient-rich water. They are rarely harmful to water life, but are of course unsightly. You can control green water by trying to achieve a natural balance by using a pond filter, an ultra-violet unit, or (as a last resort) algaecides. The natural balance depends upon starving the algae of light and nutrients, and creating conditions suited to the growth of algae consumers, such as water fleas.

Natural Balance

  • One oxygenator per two square feet
  • One snail / tadpole per two square feet
  • 60% – 70% water cover
  • No more than one inch of fish per two square feet


  • May damage underwater plants and lilies
  • Dying algae uses up oxygen causing fish death

Coagulators (Aqua-rem)

  • Binds algae cells together and sinks to the bottom of the pond to be removed by wet/dry vacuum or pond vacuum
  • Sediment must be removed or the dying algae will use up oxygen

Thread Algae

Thread algae and blanket weed are the numerous forms of filamentous algae whose masses of threads can be found tangled around plants, in cloudy masses on the pool base, forming dense mats on the pool side and causing frothy scums on the pool surface. They are unsightly rather than harmful and in balanced conditions they rarely become a problem. Thread algae can become a problem in well-lit ponds that are rich in nutrients. Most tap water supplies are rich in nitrate and this exacerbates the problem every time the pool is topped off. Fitting a biological filter will control pea-soup algae but cannot control thread algae. In filtered ponds, the clear water and lack of competition from green-water algae can actually enhance growth of thread algae. One method of control is to pull out as much of the algae as possible using a fine mesh net to scoop out the cloudy types and a stick to pull out the more entangled types. You can also reduce the nutrients and sunlight available to the algae by growing plenty of floating and underwater plants. Try to avoid topping the pond off with lots of tap water and make sure as little soil as possible washes into the pond.