How to Fix Smelly Pond Water (5 Easy Steps)

If you find your pond water smelling unpleasant, often like rotten eggs or sewage, there is likely a substance imbalance or build-up of waste causing the problem. Smelly water is generally noticed during Spring or early Summer when pond water begins naturally circulating due to the change in weather conditions. Often you’ll have a bad smell building at the bottom of your pond over the entire Winter, but the smell won’t be released and noticed until it mixes with the upper layers of pond water as temperatures rise. This is why cleaning sludge before winter sets in is an important step in “winterizing” a fish pond!

For a garden pond without fish, a smelly pond is usually not an immediate threat to the ecosystem, but should still be investigated before any underlying problems can worsen. A substance imbalance can cause problems with the natural nitrogen cycle of a pond, and this will affect a pond’s ability to filtrate water and break down harmful compounds. For ponds with fish, a strong-smelling odor around a pond is cause for concern, as it usually signals a build-up or imbalance of waste substances which can both become a major issue for aquatic organisms.

Why does my pond smell like rotten eggs?

A smell of rotten eggs is caused by a combination of carbon dioxide and hydrogen sulfide, which are the waste products of anaerobic bacteria which work in low oxygen conditions. A heavy, swampy smell is typically noticed during and after a heavy algae bloom, where new algae are growing and old algae decomposing at a rapid pace. You can learn about the different types of common pond algae and which ones are likely to cause this problem here. 

There are a few common causes of bad pond odours, and all should be taken seriously and treated if you have fish in your pond. Even in fishless waters, a bad smell is usually an early indicator of a natural imbalance that can slowly damage the ecosystem. Discovering the cause and treating the smell early will prevent major issues from occurring in the future, and will also stop the odor from becoming too potent for yourself and your guests!

Steps to Fix & Prevent Smelly Pond Water

Step 1: Provide Extra Aeration (Oxygen) to the Pond

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Stagnant pond water is not only the most common cause of pond smells, but it can also lead to a whole host of other issues. All types of ponds, with or without fish, will require a stable amount of aeration and water flow to remain healthy. Aeration provides oxygen to fish and beneficial bacteria, nutrients to plants, and movement to prevent harmful substance build-up.

Aeration can be achieved naturally with water features, such as waterfalls or fountains, or can be achieved with a physical aeration system. If you have a heavy fish-stocked pond, we recommend you aerate as much as possible! You cannot over-aerate a pond, so the more oxygen and flow you can provide to the waters, the healthier your fish will be. Having just a filter and pump is often not enough, and ponds would benefit from a fountain, waterfall, or dedicated oxygenation unit.

If you have a smaller pond or a pond that gets plenty of sunlight, you can even install a solar fountain instead of a fountain that requires mains electricity. These are great choices for extra aeration and water movement with no monthly running costs attached. They work best in smaller ponds, or as a complement to a larger aeration system in a larger pond with fish.

In winter, if your pond freezes over regularly, a de-icer should be installed on the surface to ensure gas exchange can take place and fish can get oxygen.

Step 2: Clean & Optimize Filtration & Media

Without a healthy filtration system, waste substances have no way of being effectively removed and will gradually accumulate in your pond. For ponds with goldfish or koi, a dedicated filtration unit is required to keep pond water clean and healthy. If you already have a filter system, be sure to regularly clean the mechanical media to prevent it from clogging. You could also try replacing the biological media with a more optimized variety to improve the number of beneficial bacteria present. Supplementing with beneficial bacteria products will also help reduce smells in the pond as more bacteria will be available to break down waste substances, like ammonia, which can contribute to bad water odors.

If you have a wildlife pond without fish and no filter box, you will need a good amount of pond plants and preferably a fountain or waterfall. Without a filter unit, plants will be responsible for the absorption of excess nutrients that can cause smells, so the more the better! Even in a natural pond, there will be beneficial bacteria present to aid in this process of substance breakdown, so supplementing with natural bacteria products can also help give a fishless pond a good boost in levels.

Activated carbon can also be added to the filter box of a pond to provide a third stage of filtration, alongside the normal mechanical and biological filtration. Activated carbon removes harsh chemicals that normal media cannot handle, and is also able to remove substances that contribute to and cause smelly water.

Step 3: Reduce Pond Muck, Sludge, & Waste

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Large amounts of sludge close to the water surface (i.e., pond shelves) will create an unpleasant smell similar to sewage. Small amounts of pond sludge are normal, but large amounts should be removed to prevent a build-up of substances that can be harmful to fish and cause bad water odors.

If you have a small garden pond, using a basic pond net should be enough to remove pond sludge and give the pond liner a decent clean. For larger ponds with a greater area and deeper points, making use of a pond vacuum and natural sludge breakdown products are the best solutions. Trying to remove sludge with a simple net in a large pond will require a lot of time, but a quality pond vacuum will be able to make short work of the task. Adding bacteria and enzyme-based sludge breakdown products after a vacuum clean will also help prevent sludge from returning as quickly, as the bacteria inside these products will help break it down as it builds.

If you suffer from heavy amounts of fallen debris in your pond, such as leaves, we also recommend installing a pond net or skimmer box to prevent sludge buildup over time. Regularly vacuuming and manual removal will work, but sometimes it’s easier, in the long run, to just prevent a problem before it begins! Both skimmers and basic netting should help prevent a large amount of organic debris from sinking in your pond and contributing to sludge, and you can even use both things together for maximum results.

Step 4: Control & Remove Algae Blooms

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Algae contribute to sludge build-up, reduce oxygen in the water, and compete with plants for nutrients. They can also cause unwanted pond smells if numbers get too large, and contribute to the odor of rotten eggs and sewage when they decay in large numbers.

There are many methods to remove algae blooms in ponds, and the choice would depend on the type of algae you have and if you currently have any pond fish. The safest and most effective method of free-swimming (green) algae control is by using a UV clarifier in your pond circuit. A clarifier makes use of powerful UV light to destroy floating algae within the cells, causing them to clump together and be removed by your filter system. If you have pond fish, using a UV clarifier is our recommended method of algae removal and control, and will help with water clarity by removing the green water color. Larger forms of algae that stick to the sides of the pond, such as string algae, will need to be manually removed with a net or pond vacuum as they cannot be sucked into a clarifier unit.

For other forms of algae control and removal check our detailed guide here. Make sure to read each step thoroughly, however, as some methods are not 100% safe for fish.

Step 5: Small Water Changes & General Maintenance

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Another way to help remove pond smells is with regular water changes, which will help bring fresh water and oxygen to the system. A drawback of water changes is that beneficial bacteria will be removed in the process and hard chemicals, such as chlorine, will be introduced to the pond. If you’re considering performing water changes to help reduce the smell in your pond, consider treating water with dechlorifier products to remove chlorine and beneficial bacteria products to stabilize natural levels.  Activated carbon can also be added to your filter box to help remove harsh chemicals which can be left as residue in tap water.

Water changes can certainly help reduce bad pond smells, but they may not be able to remove the cause, so the smell may eventually return. For example, a water change will not be able to reduce pond sludge from the bottom of the pond. It also won’t provide aeration to stagnant water in the long run, or stop new algae from growing. Water changes should be carried out alongside other forms of pond maintenance to ensure water odors don’t slowly return.

As well as water changes, pond equipment should be properly maintained and cleaned. Pumps need to be cleaned if there is a reduction in water flow, and water features removed of algae and debris to provide good surface aeration. Overgrown plants should be trimmed back, and dead plants and fish removed asap before they have time to decay. One of the easiest ways to prevent problems with pond smells is to simply make sure your pond and its equipment are in tip-top shape!

Common Causes of Smelly Pond Water & Odours

1) Stagnant Pond Water (Low aeration)

The number one cause of bad pond smells is due to a lack of water aeration, which usually results in a strong smell of rotten eggs. A pond needs to have some level of water movement or it will eventually begin to stagnate. Aeration provides a pond with fresh oxygen, nutrients, and water flow, which helps prevent a build-up of substances in any one area. Pond stagnation usually produces a strong smell of rotten eggs, and is often noticed during Spring when water begins to naturally circulate due to temperature and weather changes.

The smell is caused by bacteria that slowly break down waste at the bottom of a pond using enzymes. Due to the low amounts of dissolved oxygen in stagnant ponds, normal aerobic bacteria cannot break down substances as they need large amounts of oxygen to function. In the absence of oxygen, slow-digesting anaerobic bacteria take over and slowly reduce pond waste.

The problem with this process is harmful by-products of carbon dioxide and hydrogen sulfide are released and slowly build at the bottom of a pond. When Spring arrives and changes in climate occur, small movements in water mix the bottom layers of water with the top layers, releasing the harmful and smelly chemicals.

High levels of these substances are harmful to pond fish, and when they mix in Spring with upper water layers, fish may become sick and die. The hydrogen sulfide is also the chemical which produces the infamous smell of rotten eggs.

2) Poor Water Filtration

A pond without a means of water filtration will slowly see an increase in waste matter that can contribute to smelly pond water. Wildlife ponds that don’t have a dedicated filter box need to be carefully balanced with natural filters, such as pond plants, otherwise there is no method of removing excess nutrients from the water. In a natural pond, plants and algae will make use of waste build-up (sludge) and excess water nutrients for growth, acting as natural filtration systems.

For a pond with fish, a dedicated filter system and pond pump are required to effectively process the bio-load of organic waste produced. Without a filter, or with an underperforming one, harmful substances and organic matter will slowly build to dangerous levels; leading to all sorts of problems, including unpleasant smells. Ensuring your filter and pump are the correct size for your pond and keeping up with maintenance is essential for a healthy fish pond.

3) Pond Sludge (Bottom Muck)

Sludge at the bottom of a pond is normal, but if the amount of muck gets too high, you may begin to smell a problem on the surface. Pond sludge is simply all the organic matter in a pond that sinks to the bottom and begins to decompose. All ponds will have some level of sludge, and pond plants will be able to use it as a natural fertilizer for nutrients. If you have a large number of pond fish producing waste, or your pond is surrounded by trees and plants that drop leaves, sludge levels can slowly build over time.

Sludge that is close to the water surface will begin to affect the smell of the pond and become noticeable, typically producing a smell of rotting vegetation and eggs. Sludge in small amounts is healthy for a garden pond, and shouldn’t cause any problems. If you notice sludge that is thick, black in color, and slowly building, it is very likely contributing to the bad pond odor.

4) Overgrowth of Algae

Another common cause of bad pond smells is an overabundance of pond algae. Algae by itself won’t cause any major changes in odour, but when large amounts of algae die off, it’s a different story! Algae blooms are typically rapidly occurring, and as soon as algae grow, an equal number also begin to die down.

The spike in decomposing algae can place enormous stress on a pond’s ecosystem, as well as greatly reduce the oxygen content that fish need to survive. The natural pond bacteria which break down organic waste require oxygen to function, and as algae die off so rapidly, oxygen is quickly depleted. Bacteria that can’t keep up with decaying matter means harmful substances will begin to slowly build up, which can lead to potent smells and bad water odor. Read our guide on safely removing pond algae here. 

5 ) Dead Pond Fish or Plants

A final possible cause of bad pond odor, which is related to a build-up of organic waste, is the presence of dead fish or plants. This is a common cause if you have had a particularly harsh winter, or if a pond goes into winter with a large amount of waste present. Cold winters and frosts can kill pond plants, and these will begin to decay slowly until Spring arrives.

Fish may also die over winter due to lack of oxygen or a build-up of harmful gases under the surface, especially if the pond is frozen. If a pond freezes no gas exchange can take place on the surface, and dissolved oxygen needed by fish and bacteria will eventually run out.

Fish may also just die of natural causes, such as sickness or age, but still need to be removed quickly to prevent spikes in ammonia and other substances that can cause odor problems. Decaying fish will produce a high concentration of harmful substances in comparison to decaying foliage, so they need to be removed asap.