Pond gone green? What can I do?


So, we’ve had a few days of warmer weather, the sun’s just about got his hat on and your pond’s gone green!

Generally green water in a pond is  due to free-floating cells of algae feeding on the nutrients in the water and is a natural occurrence. It is only in very rare situations harmful to any fish or wildlife.  It’s not necessarily unhealthy, just as apparently clear water is not always safe actually, many fish tend to have brighter colouration when kept in green water, due to the pigments in the algae entering their diet.

If your pond is relatively new green water is to be expected as they  generally have higher levels of dissolved nutrients. These nutrients are introduced in fresh tap water and in the planting soil. Generally this green will fade of its own accord after a few weeks, as the plants and pond wildlife establish and a balance is achieved. The green can return as a slight haze for a few weeks at the beginning of each season.

More persistent problems are often associated:

  • in shallower ponds
  • ponds that exposed to continuous sunlight
  • where nutrients can wash in from surrounding soil
  • uv lamp weak or gone*
  • topping up with tapwater
  • larger fish stirring up the pool base
  • overfeeding
  • overstocking


There are many treatments available (algaecides and flocculating chemicals), these are often short term solutions but definitely are effective.

Its far better if you do use treatments to apply them later in the evening when it’s cooler, to ensure that is adequate oxygen in the water as the accepted “side effects” of the flocculating chemicals can us up valuable oxygen (warmer water has less oxygen in it). . Changing all the water will give only temporary relief, and the newly added minerals could encourage the pond to go green again in a short time.


  • Water  plants do their bit and make a valuable contribute, but an efficient biological filter will usually clear the pool of algae, in addition to improving the water quality for fish
  • Floating plants will add valuable shade to the pond
  • Reduce nutrient levels by removing some of the bottom sediments
  • Top up with rain not tap water (always treat tap water too)
  • Add barley straw. When it rots down it produces a range of chemicals that kill many types of algae. It takes about 28 days to become active and needs replacing every 4-6 months
  • With the warmer weather a natural build up of zooplankton (such a daphnia) will eat algae
  • A pump or filter with a  UV clarifier (or a separate unit) will reliably control algae without the need for additional chemical treatments


Firstly, clean the glass sleeves. If they’ve become dirty this will stop the UV from penetrating the water and dilute their effect.

Secondly, change the lamp. If your pump/filter/UVC may have a “viewing window” and you can see your light lit up, they do lose their intensity. They normally require changing every 9-12 months