Why Is My Pond’s Water Green?


When people build a pond in their backyard or garden, they envision beautiful crystal clear water with plants and fish swimming around. But what happens too often is the water turns murky and green very quickly after filling it up.

Green pond water is caused by millions of micro-organisms living in your pond’s water called algae. Algae builds up over time due to an imbalance of nutrients in the water such as an overabundance of nitrate and phosphates. Algae can be eliminated by rebalancing the ecosystem of your pond. This rebalancing can include reducing the number of fish and increasing the number of plants in your pond.

Thankfully, algae is not dangerous and can be cleared out and prevented in a number of different ways. Let’s go over all the variables involved and get your pond’s water back to be beautiful and clear again.

Does your pond look like this one?

What Causes Green Pond Water?

As I mentioned in the introduction, green pond water is caused by an overgrowth of algae in your pond. Algae growth can get out of control when there are too many nutrients in your pond and not enough aquatic life like plants to counterbalance it.

Plants need nutrients to live and pond plants derive most of their nutrients from the water in which they live. The same goes for algae, too, and when there is too much nutrients in the water and not enough things to consume them, algae flourishes.

Think of algae just like weeds in your garden. They thrive on the water and fertilizer that normal plants do and if allowed to get out of control they can take over your garden.

Fortunately, unlike land-based weeds which arrive via spores carried in the air, algae can be controlled and prevented much more easily. Or at least relatively more easily provided the appropriate steps are taken.

Too many fish can also be a common cause of algae overgrowth in a pond. Even just a few large fish in your pond can produce enough waste to fuel an algae infestation and turn your water green. This happens because algae feeds off of the nitrates that appear when fish waste is broken down.

If your water is only slightly green, it’s probably nothing to be too concerned about. All ponds and bodies of water will have some algae present – it’s completely natural and a healthy part of the pond’s ecosystem. But if there is too much excess nutrients in your water you could soon see that algae grow out of control.

A simple nutrients test should suffice to let you know if your algae is on the verge of a full bloom.

Is Green Algae Water Harmful?

Thankfully, most algae is not harmful to either your pond or the aquatic life in it.

However, some kinds of algae actually do pose some risk. Cyanobacteria, or blue-green algae, is very similar to normal green algae. Just like normal algae, they multiply in stagnant water that is rich in nutrients, especially warm water. Cyanobacteria, as with all algae, typically blooms during the late summer.

Cyanobacteria is relatively harmless by itself, but the waste that it produces can be particularly toxic. According to the CDC, cyanobacteria is known to produce the following toxins:

  • Microcystin
  • Nodularin
  • Cylindrospermopsin
  • Anatoxin-a
  • Anatoin-a
  • Lyngbyatoxin
  • Saxitoxins

Cyanobacteria related toxins are not just harmful to the life in your pond, either. Depending on the concentration of these toxins, humans can also see adverse health effects from exposure to them. Symptoms can include everything from a stuffy nose to liver failure, so be careful!

Thankfully, the normal green algae that appears in most ponds is not harmful and mostly just a nuisance.

Cyanobacteria often has a blueish green color and is potential harmful.

How To Clear Green Pond Water

Now let’s get to the reason why you probably googled this in the first place. How can you eliminate green water from your pond? Luckily, it’s really not too difficult if you know what you’re doing and have the right tools.

Method 1 – Drain The Pond

The easiest way to get rid of green water from your pond is to just start over. To do this, drain your pond using a sump pump. Once the water level is low enough you can clear out the sludge and muck from the bottom of the pool. This smelly mess is the result of decayed debris and other matter that has sunk to the bottom of the pond is fueling your algae bloom.

Once the pond bed has dried and has been cleaned, you can fill it back up with water, but consider making a few additions first such as some large rocks.

Rocks are amazing at keeping ponds clear because they serve as breeding grounds for beneficial bacteria that will help keep algae at bay naturally. This kind of bacteria sucks up the nutrients that algae needs to grow. The bigger the rock, the better.

Sometimes draining the pond and starting over is the way to go.

Method 2 – Add A Bio-Filter

Bio-filters are a kind of filter that is attached to a pre-filter. But instead of removing something from the water it actually works by adding carbon-dioxide to the water that passes through it.

What this does is promote the growth of good bacteria in your pond’s water which, like the rock method above, helps to minimize the amount of nutrients available to algae.

Method 3 – Try Using UV Clarification

UV pond clarifiers are a kind of filter that uses strong UV light to eliminate and kill free floating algae that exists in your pond.

This method is most efficient at cleaning algae in smaller ponds but will likely struggle with anything larger unless you’re using an industrial sized clarifier. Under normal circumstances, UV clarification can clear your pond within a couple of weeks.

UV clarifiers work in conjunction with other kinds of filters since they do not actually remove debris from the water. Also it’s worthwhile keeping in mind that if your algae has gotten to the point where it is all stringed together, it will not be as effective and you may need to net it out manually.

Method 4 – Try Using Pond Dyes

Pond dyes act as a kind of artificial shade for your pond in order to block sunlight. Blocking sunlight helps minimize algae growth by lowering the temperature of the water and reducing the amount of nutrients available for them to multiply.

However, please be aware that this method is design to prevent algae growth and will not do anything to remove the algae that is already there. The dye will need to re-applied every 1-2 months.

How To Prevent Green Pond Water Naturally

The best way to clear algae from your pond is to not let algae bloom to begin with. Here are a few natural preventative measures you can take.

Plant More Pond Plants

Plants are perhaps your best bet for a natural defense against unwanted algae growth.

Plants feed on the same nutrients that algae does so by placing more plants in and around your pond’s water they will be competing for these nutrients and starving out the algae.

Pond plants help reduce algae naturally.

Shade Your Pond

Placing shade over your pond works similarly to the pond dye we discussed earlier by depriving your pond of sunlight. Sunlight creates the nutrients that algae feeds from.

However, instead of using an unnaturally dye to achieve this, you can simply place some sort of a canopy over your pond’s water. You could also consider placing a larger tree next to the pond.

And if you haven’t built your pond yet, you may even want to consider building it in a shadier spot, although keep in mind there are pros and cons to doing this.

Reduce Fish

As I briefly mentioned earlier, fish produce waste that eventually gets turned into nitrate. This nitrate is exactly what algae wants to feed on before it starts blooming all over your pond.

Even just a few large fish can create enough waste to start a full scale algae infestation, so be mindful of the balance of fish and plants you keep in your pond. If you pond is on the smaller side, don’t give in to the temptation to have it teaming with fish without making sure you have ample and strong enough filtration to go alongside them.

Add Algae Eating Fish

Contrary to the last point, some fish actually eat algae. These kinds of fish are well know to aquarium owners due to their diligent drive to keep their tanks clean.

Pleco fish, Mosquitofish, Siamese algae eaters and grass carps are all great at removing algae and other debris from ponds. These kinds of fish are more commonly known as “bottom feeders”. However be advised that these kinds of fish are known for stirring up the mud and sediment at the bottom of the pool looking for food which can make the water appear dirty.

These fish love eating algae and other pond waste.

Keep Your Pond Clean

Keeping your pond clean goes without saying but you really need to keep your pond’s water free of debris like leaves and sticks that will fall in every now and then.

This debris, if left to sit on the surface, will eventually sink to the bottom and decay. Decayed organic matter provides and abundance of nutrients for things like algae to feed on.

Final Thoughts

Keeping your pond beautiful and clear of algae means keeping the delicate balance of nutrients and bacteria in your pond’s water in check.

Too much nutrients in the water means lots of available food for algae to feed on. Without proper care, this algae will bloom and be much more difficult to clean and keep under control. Some kinds of algae, such as cyanobacteria, can actually be harmful to the aquatic life in your pond as well as humans.

You can attempt to clean green algae from your pond by doing things such as draining and cleaning it, using a UV clarifier, adding a bio-filter and using pond dyes.

In terms of natural prevention of algae, common strategies include competing pond plants, adding shade, reducing the total number of fish, adding bottom feeding fish and keeping your pond clean of debris.

Hopefully this information has helped you figure out why your pond’s water has turned green with algae.

Nick

I’ve been a backyard pond enthusiast for many years and I love giving back by sharing my knowledge. It’s my goal to inform and inspire those in the process of planning and building a backyard pond.

Leave a Reply

Your email address will not be published.

Recent Posts