Why Is My Pond Water Green? | How To Clear Green Pond Water

I received an interesting question from Jane, a subscriber to my free newsletter articles who lives in Augusta, Georgia, USA.

In essence Jane was baffled as to why her pond also seemed to be murky, despite the fact that she was using a skimmer filter, pond pump and a UV light for algae control. She wanted to know how to clear green pond water (murky pond water).

This what Jane wrote ...

"Hi Sean,

I have a backyard pond that is approximately 3,500 gallons in volume and use a Fishmate 235 watt pump and a combined Fishmate 25 watt gravity discharge UV filter.

When I purchased the equipment I was assured that it was adequate for the size of my pond.

I'm sure that the system is powerful enough because the water comes out of the filter at some force.

I only have about 6 small goldfish in my pond as Herons kept eating my Koi, so I'm sure that it’s nothing to do with having too many fish in my pond.

A friend of mine suggested that I should attach a sand filter to my system as this cleaned up his pond. I tried this and the water cleared up fairly quickly.

Unfortunately as soon as I detached the sand filter the water started to go murky again within no time whatsoever.

Why is my pond water green? Have you got any advice that may be of use?

Thanks

Jane"

This is the response I sent back to Jane...

"Hi Jane,

Thanks for the email regarding your murky pond situation and how to clear green water in a pond.

First things first please don't use a sand filter in your pond ever again. Whilst they are excellent for polishing water (making the water look clean) they most definitely should not be used in a fish ponds or water garden.

The problem with a sand filter is that the filter material itself (sand) over a fairly short period of time encourages the build up of heterotrophic bacteria, which can lead to disease and fish fatalities. They also require regular backwashing and are generally not suitable because they aren't designed to handle the large biological matter and decaying matter found in a fish pond.

In a swimming pool, which is what sand filters are designed for there is little chance of the heterotrophic bacteria developing because of the chlorine added to the water; of course chlorine can't be added to a pond.

If you would like to know more about why Sand Filters Are Bad For Fish Ponds then take a look at the article relating to sand filters here ...

Anyway to get back to and to answer the question of "Why Is My Pond Water Green?" please read on...

The green fish pond water that you mention sounds to me like a build up of suspended algae. This is particularly common in summer when there is more natural sunlight and more biological activity taking place in the pond.

When it comes to how to clear up green water in garden pond a UV light is without a doubt the best way of controlling this green/brown nuisance.

Unfortunately you didn't say how powerful the UV light is. Green water algae build up in ponds with a UV Clarifier is usually down to one of the following:

  • The UV bulb needs replacing; a bulb usually last about a year and then needs to be replaced.
  • The UV wattage may not be powerful enough for the size of your pond.
  • The flow rate through your UV Clarifier may be too high, resulting in any algae not being subjected to bombardment from UV light for long enough. I notice that you mentioned that water comes out of your filter at some force.

I would advise you to take a look at the flow rate of the water being pumped through your system as I'm willing to bet that this is the culprit. It won't do any harm to change the UV bulb either.

Is your pond situated near to a flower bed that is regularly fed with plant food or fertilizer? If it is then there may excess nitrate leeching into your pond. Nitrate is of course a food source for aquatic plants and pond algae. This will encourage the pond algae to multiply rapidly leading to an increase in green pond water.

If you are looking for "how to clear green pond water" on a continuous basis then these industry leading UV Lights for both small and large ponds come with good warranties and are designed for in pond use.

Please note that when you are looking at how to clear green pond water caused by planktonic suspended algae then a UV clarifier is without a doubt the best method. If however you have a problem with blanket weed also known as string algae then a UV will not work ... You need Viresco Blanket Weed Destroyer for excellent, proven results.

Hope this helps

Sean"