Green Pond Water

When you build a new pond everything looks perfect.

Everything is clean, the water is crystal clear, water quality is good and you can see your fish happily swimming around.

Clear water is like the holy grail of all pond lovers and people spend years perfecting the balance in their ponds to achieve this.

But what happens when your new pond water turns green. It’s not because you have done anything wrong, more that things are slightly out of balance with your setup.

Green pond water is caused by an imbalance in your eco-system.

This could be down to the number of fish and waste build-up, lack of plants to naturally filter the water and remove ammonia and nitrites caused by fish waste, using too much fish food, or lack of proper filtration and beneficial bacteria to clean out your water over time.

So let’s look at some of the main factors for green pond water in more detail.

A build-up of green algae causes green water. These algae feed on waste in your pond and if left unchecked get out of control very quickly. In a matter of days, your nice clear pond teems with green water algae and looks like pea soup.

The most common forms of algae that may be found in pond water include spongomonas, euglena as well as chlamydomonas – you don’t need to worry about the names too much – just how to get rid of them and get your clear water back.

These single celled algae will turn your water green in a matter of days if left unchecked.

It makes no difference if you have a small pond or a large Koi pond or Lake. The dreaded green water will hit you at some stage so read on to find out how to stop it in it’s tracks.

Why is the pond water green?

As you pond matures a small bit there may be a build-up of fish waste, dead leaves, and other debris that has fallen into your pond.

The pond water itself might be ok but its the waste that feeds the algae and turns the water green.

If you have plants in the pond they can help with the green pond water problem, but can also add to it:

green pond water

Pond Plants – Good or Bad?

If you recently added some aquatic plants to your pond – as we all do at the start, you may have dumped them into a pot with topsoil and some compost or potting mix. This nutrient-rich soil will feed the algae and make things even worse.

The best option would be to use soil well below the topsoil layer and wrap the inside of the pot with hessian cloth or newspaper before you add in any plants.

Cover the top inch or so of the soil with pea gravel and stones to stop the soil floating away when you put the pot into the water. I often soak the pot in a bucket of standing water first to wet it up a bit and let it settle, before gently and carefully placing it into the pond – slow and steady wins the race here.

Having the right mix of aquatic plants in your pond will eventually result in clear pond water but this takes time. One type of plant you should add in at the start is an oxygenating plant. Ceratophyllum submersum or Hornwort is great, this quick-growing pond plant will help add oxygen to your pond and help combat the green water problem.

Another great plant you can use at the start to add as natural filtration is pond cabbage. Be careful though as while these floating plants provide many benefits such as filtering out waste and providing cover from direct sunlight, they grow really quickly and can get out of control if you leave them unchecked.

Whatever you decide on with your plants, and the number of plants in your pond, make sure you pot them properly and keep them under control.


A huge part of any healthy pond is filtration. Now, this can come in many forms.

If you are lucky enough to have space you might set up a natural bog filter with an array of plants like floating plants and a good amount of iris etc.

These filters use natural filtration by way of plant roots to remove all the gunk out of your water, help with phosphate levels, and cure the nutrient imbalance problems that cause green pond water.

You can jump-start your filtration by adding some beneficial bacteria to the filter once you have everything up and running.

We cover these in more detail in this pond filtration guide.

For most of us, a biological filter is the best choice because you plumb it into a pump and away you go. These filters contain biological media such as mats and balls or cylinders of plastic media that provide plenty of surface area for good bacteria to grow on.

Ammonia and nitrite are caused by a build-up of waste and rotting food or vegetation.

The good bacteria that live in your filter system bacteria feed on the ammonia and nitrite in your pond water – the same stuff that the green algae feeds on so once you get the balance right the green pond water problem starts to go away.

You have a huge amount of options available to you when it comes to biological filtration but the one thing to look out for and we see this all the time. A 1000 gallon fish pond without fish can be serviced with a 1000 gallon filter, but once you add fish into the equation you should nearly half the strength of your filter.


The amount of fish in your pond changes everything. Overstocking is a big problem for new pond owners and everybody gets excited when they start out.

Small fish from the pet shop, when let swim freely in your pond grow into big fish.

A two-inch koi under the right conditions can reach over a foot in length in a couple of years.

Less is more, especially in a new pond that has yet to settle down.

Goldfish are not too bad when it comes to waste production but koi, on the other hand, are poop machines and you can very quickly overdo it which results in green pond water.

Feeding your fish is also a problem and can add to your green water problem.

Only feed them enough food that they each within a couple of minutes or else decaying food will build up and feed the water algae turning your pond a nice shade of green. Make sure there is nothing left behind.

If there is food leftover then reduce the amount the next time and try again.


We mentioned aeration earlier when we talked about plants.

Another way to introduce air into your pond is with an air pump, a waterfall, or a fountain. Plenty of oxygen in the water helps keep things in balance and also helps circulate the water in your pond while improving overall water quality.

Good aeration alongside the correct pond plants will clear green algae in no time.

A quick way to add oxygen to the water is to fit a fountain, while some love these and some hate them, they do help with aeration and can really help turn your pond around and reward you with crystal clear water if you couple this with a decent filter.

An air pump and air stone may set you back a few bucks but its well worth the investment if it works in your design.

Feeding your fish is also a problem and can add to your green water problem.

Only feed them enough food that they each within a couple of minutes or else decaying food will build up and feed the water algae turning your pond a nice shade of green. Make sure there is nothing left behind.

If there is food leftover then reduce the amount the next time and try again.


Sunlight is one of the biggest stimulants of green pond water.

When designing your pond or planning things out try to position your pond in an area of the garden that has some shade during the day.

A pond that is blasted by full sunlight all day may start off crystal clear but you will end up a nice clear green pond in a matter of weeks or days if you don’t have any shade.

Plants can come to the rescue again here.

Having the right plants in your pond help shade the water surface from direct sunlight, give the fish somewhere to hide, eat the junk out of your water, and keep the algae under control.

Floating plants such as lilies or water cabbage are perfect in this scenario but remember to keep them under control or you will have another problem with the water turning green.

UV for green pond water

If all else fails in your quest to remove green water algae then you may need to look at adding pond UV lights to your filtration setup.

As mentioned previously green water is caused by algae and UV clarifiers kill the algae quicker than any natural solution – in the short term anyway to clear green pond water.

Modern pond filters come with these included so it’s really just plug and play for the setup.

If you are chasing clear pond water then a UV is an essential item on your shopping list.


Do not rely on the UV alone.

Yes, it may be effective enough to provide clear water for pond or water features but this does not mean the water is clean.

It may still be home to ammonia and dangerous nitrites so make sure to add a biological filter in as well.

Is green pond water bad for fish?

The quick answer to this is no.

Green in water caused by algae does no harm to your fish. It does more harm to you because you cannot see into the pond and admire your fish. A ‘clear green pond’ is more of a nuisance than anything but the fish don’t really mind.

In fact, on a hot day, the green water in pond can act as a shade from the sun if you don’t have floating plants and help keep the water temperature down a bit.

The sun is more of a problem because it helps the algae grow quicker the turns the water green in the first place.

What is more harmful to the fish is when you try and treat the green on pond problems with an algaecide or chemical treatment and cause a mass die-off quickly.

Algae, like all water plants, create oxygen and this helps feed the good bacteria in your pond that prevents algae.

If you suddenly kill all the algae then you will see a sudden drop in oxygen in your water that may cause harm to both your fish and the bacteria in your filter system.

Make sure you use an appropriate chemical (if you really have to) and follow all instructions and dosage levels to the letter.

Green pond water does not cause any harm to your fish whatsoever but which would you prefer?

Pea Soup or a crystal clear pond so you can admire your fish and aquatic plants while having a cuppa…