Help! My pond water has turned green, what do I do?

green algae in pond water

Don’t panic if your fish pond has turned green, although its not very attractive it won’t harm your fish, plus there are steps you can take to get the water clear again.

The dreaded green water that effects ponds is one of the most common problems that a fish keeper has to suffer.

Don’t worry there are several ways to get your pond back to that clear pond you desire.

What causes pond water to turn green?

A pond turns green because of an algae floating on the surface of your pond, it grows because there is an imbalance of nutrient in the pond and sunlight hitting the water.

When you have high levels of nitrate and phosphate in the water an algae bloom will appear.

The reason why nitrate and phosphate levels rise is because there is to much muck and sludge sitting in the bottom of the pond.

The sludge is a build up of organic matter from fish waste (poo), dead algae and rotting debris from leaves and grass clippings.

As this sludge decomposes it releases lots of nutrients into the water, these nutrients provide food for the algae to grow, as more and more sludge builds up more nutrients are released which feeds the algae, before long the pond turns green with algae.

Do fish cause a pond to go green?

Yes and no, fish produce waste and it’s this waste that gets broken down into nitrates which feeds the algae.

One common problem is having too many fish for the size of pond, the more fish you have the more waste is going to be released, if your filter system can’t keep up with all the excess waste your pond could turn green.

Sometimes it’s not the amount of fish in your pond but the size of fish, it’s very tempting to add say, 20 little fish into a pond, but fish grow and before long these little fish will have grown into very big fish which will produce a substantial amount of waste.

You also have to think of reproduction, goldfish may not grow as big as something like a Koi fish. but they can have lots of babies throughout the year raising the level of fish along with the amount of waste they bring.

Do pond plants have an effect on green ponds?

Yes they do and maybe not as you think, a lack of plants rather than too many plants effect your ponds water.

Pond plants consume nitrates and phosphate which is the same foods that the algae that are responsible for turning a pond green love to eat, so the more plants in your pond the more food is going to be consumed by the plants which leaves less food for the algae.

If you don’t have any plants in your pond or very few the algae will thrive because there isn’t anything else to lower the nutrient level.

Floating pond plants such as water lilies and water lettuce will reduce the amount of sunlight that hits the water, this can help reduce the algae in a pond as algae need sunlight to thrive.

How to get rid of green algae in your pond.

There are three ways to get rid of the green algae in your garden pond but which is best for you and your fish.

Don’t think your filter system will help, this troublesome green algae is so tiny it will pass through the finest of filter.

No pond will ever be free of algae, but it can be controlled with a well-balanced environment.

Using UVCs (ultraviolet clarifiers) to remove the algae.

A UVC is a unit that has a chamber holding an electric tubular florescent lamp that emits UV light, they are housed in a dark housing to protect humans eyes from coming into contact with the harmful UV light.

They work by allowing the green pond water to enter the clarifiers inlet tube and travel past the UV light which is usually encased in a quartz sleeve to stop water from getting into the electrics.

As the algae passes by the UV light it destroys the algaes ability to multiply, the algae then clumps together into particles which are large enough to be removed by your ponds water filtration system.

Once the clumped algae is trapped by your biological filter the algae will be broken down and clear water goes back into your pond.

There is a cost involved in running a UVC which needs to be taken into account plus you will have to change the UV lamp every year.

Check out our guide Best UV Clarifier for garden ponds if you want to buy one.

Natural ways to manage algae growth.

Cleaning your pond and keeping it clean through regular maintenance is one of the best ways to keep the dreaded green algae at bay.

Start by cleaning the sludge that has settled on the bottom of your pond using a long handled net or a pond vacuum.

You will need to do this a few times a year, spring, summer and autumn, remove any floating debris as soon as you see it in your pond so it doesn’t sink to the bottom and start decaying.

Using a skimmer is a good idea especially if you have a large pond, they will collect leaves and debris from the surface of the pond before they sink to the bottom also reducing the need to clean the bottom of the pond so often.

Adding plenty of plants to your pond to take up the nutrients is a great idea and also looks attractive, this results in fewer nutrients for the algae to feed on.

Add floating plants to stop sunlight entering the water, plants such as water lilies will cover part of the pond and deprive algae of sunlight.

It’s also said that barley straw will help in controlling algae, just sink a bail into your pond in the spring. Cover the bail of straw with hessian to stop bits floating to the surface.

Reducing the amount of fish that you have in a pond, especially if it is over stocked will help to control the growth of algae.

Water treatments for cleaning green ponds.

Although we prefer the natural options there are treatments that can be used to control algae growth.

Green water treatments are sometimes needed to give a boost to other treatments that are going on in your pond.

They working in different ways, there are some that will kill the algae, some remove excess nitrates that you have in your pond to starve the algae and some actually tint the water slightly to stop the sunlight penetrating the water and feeding the algae, but most use a combination of all three.

Using a water treatment is often just a short term remedy, but they are definetely effective.

Here is a selection of green water treatments from pondkeeper.co.uk and amazon.co.uk that you can use.

Why did my new pond turn green?

If your new pond turns green don’t worry, it’s usually down to using tap water that has high levels of dissolved nutrients.

Usually the green water will fade after a few weeks as the pond finds it’s own balance.

As long as you have a proper filtration system, a good pump turnover, the right amount of plants for the size of pond and use a bacteria treatment, just trust in your new pond and let it defeat the green algae in time, just be patient.

Will a water change solve the problem of green pond water?

Just changing all the water in your pond is not the best thing to do for your fishes sake, it won’t solve the problem of green algae, sure it will look good for a few weeks but the green water will return if the ponds original problems and why the water turned green in the first place are not rectified.

If you want to give your pond a thorough clean, the best way is to pump all the water out of the pond into a holding tank with your fish, then once you’ve cleaned the pond return the same water and the fish back into the pond, this will be better for your fish than fresh water, with the pond now cleaned you should notice the green water starting to clear.

If after cleaning your pond you decide to change all the water with fresh clean tap water you can add treatments to help your pond get back up and running, a better solution is to do half tap water and half original water, but still treat the tap water.

You can do partial water changes that may help, but do it little and often is my advice and never do more than a 50% water change in one go, but remember if you add tap water you are adding nutrients which algae live off, plus tap water contains chlorine which will need to be treated.

My advice for treating green algae is to not introduce new water into the pond, this will just be a very short remedy to your problem, instead treat the problem at source and give it a few weeks for the pond’s ecosystem to balance out and clean the water naturally.