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 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

The best method of preventing algae is to properly balance your pond. Too many fish, not enough oxygenators and too much light combine together into the perfect growing environment for algae. Plants are your number one algae preventers, not only by using up nutrients in the water that the algae needs to grow, but also by providing shade and oxygen to the pond. Listed below is an easy to follow method for making sure your pond won’t become an algae filled swamp.

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


Use algaecides carefully, especially in fish ponds. Regardless of which algaecide you choose, make sure to thoroughly read and follow the instructions. Using too much may result in fish and plant death, too little won’t take care of your algae. 

  • May damage underwater plants and lilies
  • Dying algae uses up oxygen, which can lead to fish death
  • Can be expensive, especially for large ponds

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.