Is green pond water bad for fish?

No. It’s quite natural for a pond to turn green. It’s natures way of dealing with excess nutrients. Fish produce ammonia when they breathe, their waste (poo) and leaves also produce ammonia. 

Ammonia is a form of plant food, along with nitrate and nitrite. All 3 will be produced in a healthy pond. It’s called the nitrogen cycle and it’s completely natural. (Click here to read a beginners guide to the nitrogen cycle).

Basically what happens is the nitrogen (ammonia, nitrate & nitrite) that is being produced are feeding single celled algae. That is the algae that makes a pond turn green.

The presence of the algae generally indicates that there isn’t enough beneficial bacteria within the pond to process the nitrogen. The good news is the algae is nature’s way of finding a balance.

The fish are quite safe. Keep reading to learn how to eliminate green pond syndrome and maintain a healthy pond.

What causes green pond water? And how to fix green pond water.

There are a number of causes of green pond water: too many fish, not enough filtration, not enough plants, water temperature and pond position. But it’s generally a combination.

As mentioned healthy pond water needs a well functioning nitrogen cycle. The backbone of the nitrogen cycle is bacteria. Specifically nitrifying bacteria.

These bacteria process harmful ammonia and nitrate (which are lethal to fish) into more harmless nitrate. 

All these compounds are food for plants. If there are no plants to absorb the nutrients then algae will grow.

So the first step is to ensure there is an adequate amount of nitrifying bacteria living in the pond. This is referred to as biological filtration as its living organisms providing the filtration.

The bacteria needs a surface to colonise. This can be rocks and pebbles, special filter media (bio-balls, ceramic noodles) or coarse sponges. The more surfaces the bacteria have to colonise the larger the size of your biological filtration.

Generally, if the biological filtration is large enough you will not experience green water. 

It’s when there is more waste being produced than the biological filter can handle that we start to see problems with green water.

The most common cause of the biological filter becoming overwhelmed is too many fish! 

A pond can go many years without issue and then all of a sudden the water clarity starts to deteriorate and turn green. This may be just the fish growing larger and producing more waste. The beneficial bacteria cannot keep up with the waste being produced.

It’s also not uncommon for pond fish to breed, this adding more load on the biological filter. The good news is this is easily fixed by finding new homes for some fish. Alternatively you can increase the size of your biological filter by adding more surface area.

Understanding some sensible fish stocking guidelines can be helpful, we have an article on how many fish for a pond.

Another cause of green pond water can be a lack of plants. Remember the nitrogen cycle takes ammonia and turns it into nitrite. Which is then converted into nitrate. There is no bacteria that will remove the nitrate.

And nitrate is still a plant food!

So without any water plants to absorb the nitrate that is produced algae will grow. Again this is natural and not harmful for the fish, just unsightly.

Adding lots of plants that can consume the nitrate will prevent algae from being needed. The only other way to remove nitrate is by removing and replacing water (water changes).

The final common cause of green pond water is the position of the pond and the water temperature. Algae like many plants likes warm temperatures and ample sun. A pond positioned in the full sun can be more susceptible to green water and string algae.

But please don’t let that deter you from placing a pond in the full sun. It’s just something to be aware of and understand. I have a very healthy crystal clear pond in full sun, it just requires more biological filtration. Click here you can read more on pond positioning.

Warm water will also deplete available oxygen. The nitrifying bacteria perform their best in well oxygenated water. During summer the use of aerators can greatly help maintain well oxygenated pond water.

So while algae likes warm water we can still have problems during cold weather. In low water temperatures below 10C (50F) the beneficial bacterias will start to decrease in activity. This means that your biological filtration isn’t operating at peak performance.

During these cooler temperatures you may experience some problems with green water. There are specially breed bacterias that are effective in cold water. They can be easily added into the pond during these times.

To learn how to fix green pond water quickly and/or naturally keep reading.

How to fix green pond water quickly.

The quickest and fish safest way to eliminate green pond water is to install a UV filter. These work by exposing the algae to ultraviolet light. 

The water that turns the pond green is single celled algae. The ultraviolet light basically destroys the cell of the algae killing it instantly.

UV filters will also kill bacteria so it’s important that light is positioned before the biological filter. That way the UV can kill the algae and the bacteria can still purify the water.

Some people are happy using algaecides to eliminate the green water. Ive used coptrol (ebay link) to treat string algae and it works great and is fish safe. You only need to use a tiny amount so it will last a really long time.

Most people have no idea how many litres or gallons their pond holds. If you’re one of them you can read my article on how to calculate pond volume.

If you don’t mind getting your hands dirty another effective and fast method of removing the green pond water is wrapping a milk crate in quilt batting and placing your pond pump inside.

The water will be sucked through the batting and the algae gets stuck in the fibres. It is messy and can take a few goes replacing the clogged batting, but it works fast and is chemical free.

While all these methods will fix the green pond quickly its only a temporary solution, well apart from the UV light that could be run continuously. But they only mask the original problem, which is excess nutrients.

Keep reading for more natural and long term solutions to eliminating green water.

How to fix green pond water naturally. 

The answer to fixing green pond water naturally is really very simple. Adding more beneficial bacterias and plants is the solution.

The regular addition of beneficial bacteria to your pond is incredibly effective at maintain a healthy population at all times. 

Also adding many different types of plants: submerged, marginals and floating. Choosing plants that are active during different times of the year will also ensure something is always consuming the nitrates.

My favourite way to provide crystal clear water is the use of a bog or wetland filter.

I have many written articles, as well as youtube videos raving about the benefits of bog filters.

The use of rocks and gravel in the filter provides an incredibly large amount of surfaces for the bacteria to colonise, while planting the plants directly into the gravel allows the roots to soak up the available nutrients.

Theres so many benefits to these kinds of filters.

  1. Allows the plants to be positioned outside the pond itself. This increases the area available for the fish and is particularly helpful if your fish like to eat your pond plants.
  2. The fibrous roots act as another layer of filtration by trapping the solid waste before it return to the pond.
  3. The look completely natural and blend seamlessly into the landscape.
  4. They require very little to no maintenance if designed well.
  5. Easy to add to an existing pond.
  6. Perfect for all ponds wether large or small.
  7. With so much filtration going on they increase the number of fish you can keep.
  8. Require no added equipment- can be feed using your existing pump.

If you would like to learn more and potentially build your own bog filter read this article on how to build a natural pond filter.


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Hi, I'm Kev. My pond and water garden started with simple aquariums. I have created many ponds and water gardens around our home: Fish ponds, Aquaponic systems, grey-water wetlands and bog filters. My favourite topic is water filtration.

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