Why is My Pond Water Red in Color (And How to Fix it)

Why is my Koi Pond Water Red? Guide to Clearing Red Water Problems

Organic debris, such as red leaves and algae, can leech tannin which makes water appear reddish in color.

We subconsciously associate the color red as a warning sign in our daily lives – but should it also worry us if we see it appear in ponds? Although reddish water problems are not that common, they can still affect a wide range of ponds, especially those which suffer with heavy debris fall throughout the year. The color red is almost always caused by the leeching of tannin (organic color) into the water from decaying organic matter. The most common culprits are leaves, especially those with red or purple foliage, such as Japanese maples, red maples, and purple beech trees. All of these trees shed leaves in autumn months, and if the leaves find their way into the pond, they’ll eventually decompose and tannin will leak into the water – giving it a red appearance!

The color may seem visually appealing, but it shouldn’t be ignored, as any changes in water clarity can be a warning sign of growing imbalances which need to be investigated. Tannin build up in ponds can sometimes mean there is excess waste which beneficial bacteria are struggling to breakdown, or could be a sign of a leak in the pond liner leeching sediment. The color could also be caused by specific types of algae or algae blooms, which can both place a large amount of stress on the eco-system if left unchecked.

The color red can sometimes present as a brown color initially before shifting to a more reddish hue. If you notice a brown color to the water, it is often caused by similar things as red water, and can also be treated in a similar fashion as described in this article!

What Causes Red Pond Water to Occur? 

1) Organic Debris Leeching Tannin (color)

The most common cause of reddish ponds is the leeching of organic colour, called tannin, into surrounding water from decomposing matter. Although any type of debris can cause tannin, if your pond is looking particularly red in color, it is likely being produced by specific debris, such as maple leaves. Regular leaves will often produce a more brown color, whereas certain red leaves will create a red or purple hue as the decay on the pond floor.

2) Sediment Leaking into Pond

Although less common than tannin, if you have damage to the pond liner or there is heavy rainfall, sediment can find its way into the pond and change its color. If you live in an area where the soil is heavy in minerals, such as iron oxides and copper deposits, even small amounts of sediment can cause a reddish tint to the pond water. The sediment can find its way in gradually through a leak in the pond liner, or can be washed into the pond after a period of bad weather.

3) Red Algae & Algae Blooms

A final possible cause of reddish water problems can occur due to algae blooms or even the growth of red color algae. When algae blooms and dies its foliage (flowers) can leech tannin as they decay on the pond floor. Likewise, if you have red algae growth in your pond it can create the illusion of red water as it reflects light on the surface water, while also leeching tannin as it dies off, similar to regular algae. Red algae may be more common in oceans, but since it’s nicknamed the “red tide“, you can be sure it’ll make your pond  red if it finds its way into your water!

Is Red Water Dangerous to Koi and Goldfish?

Tannin (color) is not dangerous in itself, but can be a sign of a growing water quality problem.

The tannin which causes pond water to change color is not directly dangerous to fish, but the underlying cause certainly could be if left unchecked. Both goldfish can koi thrive in high oxygen conditions where water quality is kept to a high standard, and if your reddish water is being caused by a build up of debris, both of these aspects could be impacted.

Beneficial bacteria in your pond and biological filter media will be responsible for breaking down the harmful substances produced from decomposing matter into less harmful components – a process called the nitrogen cycle. If waste levels spike or rise too high, however, bacteria may not be able to keep up with demands and harmful substances, such as ammonia, will begin to rise. As well as this, beneficial bacteria require large amounts of oxygen to function, and in heavy waste conditions they’ll be using much more in an attempt to process the excess substances. This means that less dissolved oxygen will be available to pond fish, which will gradually cause them to become stressed and even sick.

Just like general debris build-up, algae has a similar effect on ponds, with blooms rapidly growing and dying in a short period of time causing large spikes in waste which bacteria struggle to process. If your red water is being caused by tannin from decaying debris or algae, it should be properly controlled as it’s an indicator of an overloaded eco-system.

If the cause is due to sediment leeching into the pond, the heavy minerals can interfere with pond treatments and medicines. Leaks to the pond liner will also cause the water level to gradually drop, which, although not immediately dangerous, can be a nuisance if you have a heavy stocked fish pond.

How to Fix Red Pond Water & Stop it Coming Back (Easy Steps)

Step 1: Remove Sources of Tannin/Debris

Pond vacuums are a quick and easy method to clean out large amounts of pond sludge in fish ponds.

The first step to fixing red water problems is to ensure that waste and bottom sludge is kept to a minimum; especially if you suffer with heavy debris fall. If your pond is in close proximity to tress, such as maples or beech, the red color is likely being produced by tannin from decomposing leaves which have sunk to the bottom of the pond floor.

To resolve this issue and lower the amount of tannin being produced, we recommend cleaning out bottom muck with a pond vacuum or wide-brim pond net. Pond vacs are a great choice for larger ponds as they make cleaning much faster and don’t kick up as much muck which can make water appear dirty for a short period. If you have a very small pond, however, a regular pond net should be sufficient to scrape the pond floor and remove small amounts of bottom sludge.

After removing a good quantity of sludge, you’ll also want to try to prevent the debris accumulating again in future or it’ll be a never-ending cleaning battle! Since red water is almost always caused by fallen leaves, installing quality pond netting or a heavy-duty pond skimmer system are good choices. Netting should have fine mesh holes to catch the maximum amount of debris, and a skimmer should have a good size leaf basket to make maintenance easier. If you have any overhanging trees, and to make cleaning a lot easier, we suggest cutting these back to reduce the amount of foliage fallout in autumn months.

Finally, after the excess waste is removed, you can consider supplementing with a beneficial bacteria product to give your natural populations a boost in productivity!

Step 2: Prevent Sediment Leeching into Water

If the problem is caused by sediment leeching into the pond, you may have damaged or insufficient pond liner.

If you don’t have problems with organic waste or any foliage nearby, the red water could be caused by sediment leeching into the pond from an external source. We find the most common cause would be soil washed into the pond during periods of heavy rainfall, which then becomes stirred up and colors the water.

Luckily, this type of red water is usually just temporary, and as the sediment settles on the pond floor, the particles which are causing the red color will also disappear. However, if the water is becoming very red in color, you’re likely living in an area with heavy mineral deposits (i.e., iron oxides), which are causing the drastic color shift. Although not immediately dangerous, a gradual build up of mineral can eventually become a problem, so we recommend cleaning the sediment from the pond floor with a vacuum after a period of heavy rain. To prevent it happening in future, you should consider raising the surrounding embankments by adding more soil, stones, or wood and then attaching more pond liner to help prevent water washing over the top into the pond. You need to also check that the liner is water-tight in all areas around the embankments, as sometimes it can slip below the water line in sharp corners. This issue is more common with ground-level ponds, and unlikely to happen with raised ponds which sit higher than the sediment level.

If you already have a raised pond, the issue could be damage to the pond liner which is slowly leeching sediment over time. Depending on the size of the pond, breaks in the pond liner can be difficult to spot, but we have a separate guide here to help identify and fix the problem. Most leaks can be repaired with cheap repair kits and some DIY know-how, but in more extreme cases, it’s better to just replace the liner altogether.

Step 3: Prevent Algae Growth & Blooms

UV Clarifiers are very efficient at removing free-swimming green algae, but can’t remove larger forms, such as string algae.

Green swimming algae can be controlled with a UV clarifier which destroys the algae at the cellular level and does not interfere with water quality. After the algae has been destroyed, it’ll begin accumulating inside your filter media which means it’ll require more regular cleaning until the algae problem as subsided. Larger forms of algae, such as string algae, can be removed manually with pond nets or vacuum cleaners as UV clarifiers can only deal with free-swimming algae particles. Using a combination of UV clarifier and manual removal should be sufficient to prevent most forms of algae blooms which contribute to sludge, but what about red algae?

Red algae is not very common in ponds, but just like string algae, UV clarifiers won’t be able to remove it as this algae tends to grow in larger bunches. The best way to prevent red algae would be a combination of manual removal (vacuum/net) and the addition of a natural deterrent; such as pond plants (long term) or a natural algae remover (shorter term). Natural algae treatments contain a specific form of algae which pond organisms love to eat, and as it grows it competes with the nuisance algae for nutrients and slowly starves its growth. As the special kind of algae grows, it attracts micro-organisms which bloom and eat the algae, and then other organisms, such as fish, will naturally feed on the smaller ones – creating a natural cycle!

During any algae bloom you should ensure your water filtration is in good order and you have plenty of aeration so your fish have plenty of oxygen content. After the bloom has died down, the bottom of the pond should be cleaned to prevent a large spike in waste and a reduction in oxygen and water quality.

Step 4: Supplement with Activated Carbon (Removes Tannin)

Activated carbon is very effective at removing organic tannin, and can be used to reduce red water after the cause is resolved.

After fixing the source of the red water, you can then remove any lingering tannin (color) using an activated carbon treatment. Activated carbon works via a process called absorption, which allows the carbon to neutralize organic substances on its surface from the surrounding water. Activated carbon works great for removing all sorts of organic pollutants, such as pesticides, fertilizers, perfumes, chlorine, and organic colors. The carbon treatment can be added to your filter box as a third-stage of water filtration, and will slowly remove tannin as the water passes through the carbon pieces. In-fact, activated carbon is such a useful treatment that many fish keepers, especially koi keepers, tend to use it year-round to improve water quality, water clarity, and fish health.

Sadly, even though activated carbon is great at removing tannin and making the pond appear normal again, it often can’t solve the underlying issue. Before adding activated carbon treatments, we recommend first identifying the cause of the red water (i.e., debris/algae), and then adding carbon as a final step to remove the lingering colour.

For our full guide on how to make the most of activated carbon and why it’s so great for ponds, check our dedicated article on this here!

Step 5: Test Pond Water Quality for Problems

Finally, as with all changes in water clarity, you should test your pond water to ensure your water quality is within safe parameters; especially if you have fish! Depending on the cause of the red water, you could also be suffering with high ammonia caused by waste, or a fluctuating pH/KH reading caused by mineral build-up. A wide range water test kit can be used to determine water quality, and should be carried out before and after treatments to make sure the issue is being resolved.

If you have had major issues with debris leeching tannin, or a large algae bloom, you can expect some higher readings before the pond system settles. Water testing can be carried out every few weeks to determine if quality is returning to normal, and if not, further steps can be taken to fix underlying issues based on the results.

For more information on testing water quality and the ideal parameters for fish ponds, be sure to check out our water testing article here.


Pond Informer (Chris)

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners.

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