Repairing Cracked or Leaking Concrete Fish Ponds

Author

Edd Walker

Bachelor of Science (BSc) in Chemistry

About the author

Edd has 10+ years of expertise in the research, development and testing of lime-based products.

How to repair a cracked or leaking concrete pond or fish pond

Being outdoors offers many benefits, and for many, that can be simply spending some time in a garden. So it’s no surprise that water features and fish ponds have become popular garden additions, having a calming effect on the mind and body, especially when the sun occasionally decides to shine here in England.
While a certain level of natural water evaporation is expected, as many fish-pond keepers can attest, nothing can ruin the peace of garden space quite like a sudden or inexplicable drop in water levels.

Do you suspect your concrete pond has a crack? Learn what to look for, your options, and how to start repairing cracks to get your pond back in shape.

Identify the cause of your pond crack or leak

Before tackling how to fix a leak, it’s best to identify the cause, as preventing damage is always easier than repairing damage. Begin with the most straightforward causes of water loss. Check for wear and tear across the various areas of your pond. This can include pond liners, walls, or any relevant plumbing connections to locate the source of the leak.

Assess the crack damage

If the suspected crack is indeed the cause of the leak, the next step will be to determine the severity of the damage. Cracks in ponds can range from small hairline cracks to large cracks that can run through the entire structure. For small hairline cracks, a simple patch repair might be suitable. For large cracks that threaten the overall structure, a full recoat of the pond with mortar is likely a worthwhile course of action.

Keep fish and aquatic plants safe

Once the crack in your pond is assessed, the next step is to determine what to do with your fish or aquatic plants, like waterlilies or hyacinths, while you make your repairs. Depending on the location and size of both the crack and the pond it may be possible to keep the fish and plants in place while you complete a simple patch repair.

In most instances, the fish will likely need to be temporarily relocated to a safe holding tank or similar while you make your repairs. If you are in doubt about how to manage your fish during the process, contact your local aquatics shop, which can advise you.

Repairing the pond with Prompt Natural Cement

Prompt Natural Cement is made from clayey limestone from the Grenoble region of France. Its quick-setting properties and excellent adhesion make it ideal for speedy and waterproof repair in a fish pond’s dry or wet environment. Depending on the pond and the type of fish, you may undertake a patch repair with or without draining the water from the pond. Below are two methods, depending on the amount of water present in the pond.

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Determine the method of repair

If you want to keep the water in the pond during the repair, you will need to consider the water’s pH levels and the sensitivity of your fish. A small amount of calcium hydroxide will be released when repairing the crack with Prompt Natural Cement. This effect is minimal for larger ponds of 10 cubic meters or more. However, this effect will be more prominent in smaller ponds of only a few cubic meters, which can have more unstable water parameters and be more susceptible to the impact of changing pH levels.

When in doubt, consult a professional for additional guidance or proceed with repairs after the pond is drained and the fish are secured in another temporary vessel. After your repairs, monitor your pond’s pH levels with test strips for six months to ensure the quality of your pond water.

Method 1: Repairing an empty pond with a full recoat

When repairing a crack, the ideal repair is to remove the fish to a safe temporary holding tank or similar and drain the pond. With the empty pond, you can see all potential areas at risk of failure, and application and finishing will undoubtedly be more straightforward without water. Further, you can better ensure a consistent and waterproof background by coating the entire surface. If you cannot drain the pond and safely remove your fish to a temporary holding tank or similar, you’ll need to proceed with method 2, patch repairing a filled pond below the water line.

To repair a pond with Prompt Natural Cement below the water line, you will need a 1:1 mortar with good-quality 5mm sharp sand suitable for use as a pond liner, you’ll need to apply this as two layers of at least 10mm each with a key between the layers. Here at Cornish Lime, we stock a range of sands in various colours and textures. The best sand we stock for this type of work is the CLS28, a 5mm coarse washed flint sand. We would strongly recommend going with a good quality sand, similar to that used for render basecoats, to help reduce the risk of shrinkage. Browse our full range of coarse sands.

Step 1: Gather your materials & tools

Once your materials are selected, gather your Prompt Natural Cement, Tempo, sand, and tools and prepare everything nearby before mixing your material. You’ll likely need a bucket for mixing, a hawk for holding the material, and a trowel to apply, level and finish the mortar. Always remember safety when working with construction materials. We recommend a set of safety glasses and a set of waterproof gloves.

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Step 2: Brush out any remaining biological materials

Brush out the pond to remove algae, loose silt or sandy materials, and allow the water to settle again so you can clearly see the area to be repaired. A standard cleaning churn brush or similar is ideal.

Step 3: Quickly and thoroughly mix the material

As Prompt Natural Cement is quick-setting, you’ll want to work in small batches, carefully but quickly mixing the amount you can apply and finish within the 45-minute setting time with Tempo. The consistency of the mix should be stiff but usable.

Step 4: Apply the Prompt Natural Cement

Once your material is thoroughly mixed and has a usable consistency, apply the mortar in two coats to a total thickness of at least 20mm.

Step 5: Leave the freshly applied pond empty for a week

Once your freshly applied coat is complete, leave the pond empty for a week and clean down the newly applied mortar before refilling.

A quick top tip

For an extra layer of assurance, wash the freshly recoated pond with Tartaric Acid shortly after the Prompt Natural Cement has set. This process will neutralise any potential alkaline release, a method commonly used in France for wine storage tanks. If this is your planned route, get in touch for detailed application guidance.

Once the week is complete and the freshly coated pond is washed down, reintroduce your fish and other aquatic plants back to their newly repaired home.

Method 2: Patch repairing a filled pond below the water line

To repair a pond with Prompt Natural Cement below the water line, you will need a 1:1 mortar with good-quality washed sand. When choosing a sand, ensure the sand will suit the size of the gap that needs filling.

As a rule of thumb, the maximum grain size should be approximately 1/3rd to 1/4th of the width of the gap. Before mixing the material, check the area around the damaged section to ensure there are no further debonded or loose sections and remove any that are present.

Step 1: Gather your materials & tools

Once your materials are selected, gather your Prompt Natural Cement, sand, and tools and prepare everything nearby before mixing your material. See method 1 for recommendations for tools and PPE to help you get started.

Browse our range of tools and PPE

Step 2: Brush out any remaining biological materials

Brush out the damaged area to remove algae, loose silt or sandy materials, and allow the water to settle again so you can clearly see the area to be repaired. A standard cleaning churn brush or similar is ideal.

Step 3: Quickly and thoroughly mix the material

As Prompt Natural Cement is quick-setting, you’ll want to avoid mixing more material than you can reasonably apply and finish into the crack within the two-minute setting time. The consistency of the mix should be stiff but usable.

Step 4: Apply the mortar to the crack

Firmly press the mortar into the crack using a suitably sized tool and hold it there until it starts to set, then quickly smooth the entire surface down to eliminate any gaps at the edges of the repair. The RST Trowel End & Square Small Tool or the RST Leaf End & Square Small Tool are two potential options that should suit the majority of applications.

Summing it up

Once a potential leak is identified, take the time to thoroughly inspect your pond and the surrounding area to determine the cause of the leak. If your suspected crack is indeed the issue, proceed with either full recoat or patch repair. Which might work best for your project will depend on the nature of the damage and if you can safely move your fish and drain the pond to get the repair in place. Each project is unique, so when in doubt about the best approach, get in touch with a professional.

The information provided by Cornish Lime is for informational purposes only and does not amount to a specification. Every project is unique, so please consult a professional before undertaking a project. Use of this site and reliance on any information on the site is solely at your own risk.

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FAQs

Any container that holds water can eventually leak. The causes of leaking concrete ponds can range from poor construction, ground movement to rouge tree roots.

Our go-to product for sealing fish ponds is Prompt Natural Cement 1:1 mortar mixed with natural sand. While other options are available on the market, ranging from standard concrete to epoxy, we favour the fast-setting, corrosion-resistant, and waterproof qualities of Prompt Natural Cement, making it a top choice for us.

There are a host of materials that can be used for waterproofing a concrete pond. These can range from polymers and plastic liners to applying a fresh coat of waterproof mortar. Each has pros and cons, and which might work best for the pond depends on several factors, such as the pond’s condition, budget, and aesthetics.

When the original concrete fails, multiple methods exist for repairing a cracked pond. In short, you can either patch repair the crack or apply a new, fresh layer of mortar across the entire pond. Which might work best will vary depending on the pond’s condition, the crack’s severity, and whether you can safely drain and remove any fish or aquatic plants to make the repair.

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