A newly installed pond starts with crystal-clear water. Problems such as green water, bad odours, and chemical imbalances can happen over time, but what causes them, and can they be fixed?

In this guide we will cover how to identify and resolve typical water issues for everything from large Koi ponds to small wildlife ponds, so you can have a healthy ecosystem.

Green water (algae blooms)

Green water is a very common issue, especially during the summer months. This murky coloured water is caused by an overgrowth of algae, which thrives in warm ponds, consuming excess nutrients.

Quick facts:

  • Caused by: sunlight and overfeeding
  • Symptoms: murky green coloured water
  • How to fix: provide shade on sunny days, reduce feeding, install a UV clarifier.

Green water treatment is a fast acting but temporary solution, but fitting a UVC is more effective in the long term. These units kill floating algae, clearing your water constantly, without the expense and effort of frequent treatment dosing. Check out our range of ultra violet clarifiers that you can easily integrate into your pond setup.

You may also choose to provide shade to your pond, either using floating pond plants, planting trees, or installing a shade sail or pergola.

Cloudy water

Cloudy water may either have a mechanical or bacterial cause. Suspended debris in new ponds or after maintenance may leave your pond grey in colour. However, biological imbalances cause bacterial blooms, giving your water a milky appearance, often when there's a drastic change in water chemistry or an excess of organic materials.

Quick facts:

  • Caused by: fine debris, bacterial blooms, poor filtration
  • Symptoms: murky or milky water, low clarity
  • How to fix: improve filtration, add beneficial bacteria, use a flocculant, and vacuum the pond

You may need to install a more powerful filter to clear the cloudiness, ideally a larger one with more mechanical media. The additional media can trap more debris. Vacuuming the pond and adding a flocculant can also be beneficial, and in the case of bacterial imbalances, introduce bacteria to restore stability to your water by breaking down organic matter.

Discoloured or yellow water

The most common cause of yellow or brown discoloured water is rotting organic matter such as leaves, dead algae, and fish waste, or by tannins from plants and soil, 

Quick facts:

  • Caused by: decaying leaves, algae, leftover food
  • Symptoms: yellow or brown coloured water
  • How to fix: complete a partial water change, and add activated carbon to the filter

Regularly changing water aids in diluting the discolouration. Activated carbon can also be added to your filter to absorb the stained colour and leave your water looking clear again. Be sure to regularly skim leaves using a net before they settle and decay to prevent discoloured water in future.

Leaves being skimmed out of garden pond using a pond net
Regularly skimming leaves greatly improves water clarity

Foamy Water

Foam on the surface of your pond often means an organic waste build-up and poor water quality. This can commonly be after algae treatment, but can also signal health issues with your fish. Foam may appear due to overfeeding or shedding excess mucus due to stress or parasites.

Quick facts:

  • Caused by: waste build-up, overfeeding, algae treatments, fish stress or parasites
  • Symptoms: persistent bubbles or foam on the water's surface
  • How to fix: reduce feeding, improve surface skimming, and add an anti-foam treatment

It is highly recommended to conduct water tests after noticing persistent foam. A partial water change is beneficial for foamy water and adding anti-foam treatments can provide quick results. Be sure to make sure your filter system is sufficient for the size and waste load of your pond.

For long-term results, check our our range of pond skimmers to prevent unsightly foam on your pond's surface.

Smelly water

A foul odour from your pond is almost certainly a build-up of decomposing waste. Rotting leaves, food, fish waste, and sludge is a leading cause in unpleasant smelling water.

Quick facts:

  • Caused by: rotting leaves, leftover food, low oxygen, ammonia build-up
  • Symptoms: bad smelling water
  • How to fix: remove debris, complete a partial change of water, add beneficial bacteria and carbon

Removing as much waste as possible with a net or vacuum during regular maintenance, or when you notice a build-up, is key in preventing bad smells from developing. Adding activated carbon to your filter will aid in neutralising odours, and adding bacteria will help to breakdown waste naturally.

Garden pond with autumn leaves littering the surface and the bottom.
Decaying leaves are a common cause of bad smelling pond water

Ammonia and nitrite spikes

High ammonia or nitrite levels are dangerous for fish, often occurring due to overfeeding, overstocking, or in newly set up ponds. Filtration issues can also be a strong factor in causing spikes, such as biological filtration not yet being established, or being removed accidentally.

Quick facts:

  • Caused by: new pond syndrome, overfeeding, lack of beneficial bacteria
  • Symptoms: gasping fish, red gills, lethargy
  • How to fix: temporarily stop feeding, do a partial water change, add beneficial bacteria, increase aeration

It's important to clean filer media in pond water rather than tap water, and stock fish slowly in a new pond to allow the filter time to mature, adding a bacteria pond treatment to kickstart the process. Be sure to regularly test your water to check for spikes, and consider adding an air pump or waterfall to better oxygenate your water.

High nitrate

Nitrate is far less toxic than ammonia or nitrite, and is a natural by-product of the nitrogen cycle. In high levels, your fish will become stressed, and algae growth will increase.

Quick facts:

  • Caused by: natural by-product of waste breakdown
  • Symptoms: algae growth, less active fish
  • How to fix: regular maintenance, add live plants

Regular maintenance, such as changing water, will lower nitrate levels quickly. To prevent future spikes, add plants such as fast-growing floating species to reduce excess nutrients naturally. Be careful not to overstock your pond, and feed fish only with as much as can be eaten within a few minutes.

Pond with algae growth
Algae growth can be a symptom of Nitrate levels that are too high

Low pH

Low levels of pH is known to be caused by acidic water or rainwater, or decaying waste. This disrupts the natural balance of the water and can affect biological filtration, causing harm to the health of the fish. 

Quick facts:

  • Caused by: acidic tap water or excess rainwater
  • Symptoms: stressed fish, low levels on test
  • How to fix: use a pH buffer

If you notice the pH levels are too low during regular water testing, it's important to raise it to a stable level. The typical level is usually between 7.0 and 8.0 for most fish, and pH buffers can be used to raise the levels safely. You may want to check your water to prevent future issues, as maintaining stable pH levels is essential for biological filtration and fish health.

High pH

High pH and excessively alkaline water can be caused by materials in or around your pond. This is typically caused when concrete or masonry is entering the water.

Quick facts:

  • Caused by: concrete leaching into water
  • Symptoms: stressed fish, high levels on test
  • How to fix: remove any unsealed masonry and change water

If your pond water test kit shows a high level of pH, usually above 9, check for any unsealed concrete that may be in or around your pond. Seal all exposed surfaces and change the water to restore the levels to normal.

Regular testing with a water test kit is always recommended to keep the healthiest pond ecosystem.

Low KH (Carbonate Hardness)

Low levels of carbonate hardness affects the water buffering capacity, which increases an instability in pH levels. In areas that have soft water or lots of rain, KH can easily drop too low.

Quick facts:

  • Caused by: soft water and rainwater
  • Symptoms: unstable pH levels and a reduced filter performance
  • How to fix: add a KH buffer regularly

Regularly adding a KH buffer will aid in maintaining stable levels, improving biological filtration. Pond buffers are ideal for use after changing water or heavy rainfall.

Low oxygen

If your oxygen levels get too low, this can be fatal to your aquatic life, especially in warmer weather when water contains less oxygen naturally. Maintaining oxygen levels is a key factor in keeping your fish safe and healthy.

Quick facts:

  • Caused by: poor circulation, hot weather, dense planting
  • Symptoms: fish gasping at the surface, slow movement, congregating near moving water
  • How to fix: add an air pump, waterfall, or fountain, and thin out surface plants

An air pump and airstone is perfect for circulating oxygen in your water. Fountains and waterfalls aren't just decorative, they also aid with increasing oxygenation in water. Plants that cover more than a third of the water's surface can reduce oxygen levels, so consider thinning them out and even adding oxygenating plants.

High phosphate

Phosphate tends to get into your pond through changing water or even fish food, acting as a fertiliser to fuel rapid algae growth.

Quick facts:

  • Caused by: tap water, fish food
  • Symptoms: algae blooms, green and cloudy water
  • How to fix: use a phosphate remover, add plants, change pond water regularly

Phosphate removing resin or media has been specially designed to be added to your filter, reducing high levels. This media provides quick results and long lasting effects, allowing you peace of mind. Planting some fast growing plants, such as hornwort, will naturally consume phosphate, and don't forget to carry out regular maintenance.

Algae being removed from a pond
Algae blooms can be due to high phosphate, which is easily resolved with the right media

Chlorine and chloramine

Tap water chemicals like chlorine and chloramine are very harmful to pond fish and beneficial bacteria. Even small amounts of these elements can cause stress, or kill the colonies of bacteria in your biological filter. 

Quick facts:

  • Caused by: untreated tap water
  • Symptoms: stressed fish
  • How to fix: use a tap water conditioner or inline dechlorinator

Be sure to always dechlorinate any new water you are adding to your pond. Liquid conditioners are very effective, or you may choose to consider an inline dechlorinator for ease of use. 

Fixing your pond water issues

Many issues you may encounter with the water are interconnected, you may find one problem left untreated quickly causes another. Regular water testing and frequent maintenance are key for maintaining the quality of your water, providing a safe environment for your pond life, allowing you to prevent most issues before they start.

Our help guide on how to achieve crystal clear pond water provides helpful tips to maximise the clarity of your pond. For more pond care essentials, check out our full range of pond treatments, pond pumps, and pond filters to keep your pond in the best condition.