What Pond Liners are Best for Koi Ponds?

Proper pond liner selection is crucial for koi ponds. While many plants will grow in practically any kind of liner, fish are much more sensitive to chemicals that can leach out from specific flexible and rigid materials. Other materials like concrete rarely leach any problematic chemicals but may come with other disadvantages like difficult cleaning and high cost. Some pond owners hope they can simply spread a layer of mud or clay over the soil and start adding water. You need an impermeable, fish and plant safe, flexible liner material from BTL Liners to build a koi pond that performs properly for years to come.

Why Liners are Essential for Koi Ponds

A solid pond liner plays many roles in the koi pond, offering advantages like:

  • Easier algae and sludge removal when the base and sides of the pond need cleaning
  • Eliminated seepage; protecting you from large amounts of water loss
  • Protection from chemicals or minerals that could leach into the water from the soil or groundwater around the pond
  • Clearer water, since there’s no thick layer of mud or clay to stir up
  • Protection from insect, aquatic, and animal pests that can burrow in from the sides or underneath
  • More thorough soaking, when medical treatments are added.

Rigid vs Flexible Liners

If you’ve only shopped for materials for backyard ponds at local hardware stores, you may think that your options are limited to pre-shaped rigid liners. Even if you’re aware of your options for flexible materials, you might wonder if there’s some benefit to choosing a rigid liner instead. However, it’s the flexible liner that offers many more benefits to the koi pond owner. The disadvantages of rigid liners include:

  • More ground preparation needed to create a perfectly level and even edg; which isn’t necessary for flexible liners
  • Greater chance of cracking due to the freeze and thaw cycle or falling debris like tree limbs because of the rigid material
  • Very few options for size and shape, leading to cookie cutter ponds that are hard design with a natural appearance
  • Extra work required for hiding the edges of the pre-formed pond liner with dirt, sand, and stone
  • No customization options
  • Short lifespan that leads to replacement after just a few years.

Unless you only want a very small koi pond for a pair of fish, you’ll need a flexible liner to build a large enough space. Pre-formed liners typically hold a maximum of a few hundred gallons of water. Yet, a pond with a flexible liner, can be designed to hold hundreds of thousands of gallons of water and hold hundreds of fish.

Materials to Avoid

There’s a wide range of flexible materials that might appear to work well for ponds but actually feature serious drawbacks. Some of these materials could kill your fish suddenly, while others simply cost too much, have a short of a lifespan, or make maintenance difficult. Liner materials to avoid include:

  • EDPM; a material primarily intended for roofing use. Plasticizing chemicals in the mixture can leach out and poison your fish and plants unless you find a EDPM liner specified as fish-safe. The material also tends to break down rapidly due to intense UV exposure in ponds.
  • PVC; is flexible and affordable but can contain plasticizers that are toxic to fish.
  • Liners with anti-fungal or anti-algae coatings. These chemicals are often damaging to fish and can reduce plant growth so there is little left for the koi to eat.
  • Repurposed tarps; when uncertain about the type of material there is a high potential for leaching toxins.
  • Concrete without a flexible liner barrier; inevitably develop tiny cracks which let water escape, in addition to the seepage that occurs to the natural capillary action of the material. In contrast, flexible liners keep this moisture from escaping the pond.


Since RPE, HDPE, and LDPE materials have all been discussed as fish-safe and durable enough for pond use, it’s time to narrow down your choices between the three. HDPE has a high tear resistance along with RPE, but HDPE is much heavier and harder to install than RPE. HDPE also doesn’t have the same high level of UV resistance that RPE or LDPE boast. LDPE is UV resistant but a little prone to tearing. All three will work, with LDPE and HDPE performing best when combined in products like BTL Liner’s AquaArmor. Their RPE liner, ArmorPro, is another good option for larger koi ponds where thicker liners and greater damage resistance is needed. HDPE and LDPE liners are generally more affordable, but RPE liners last longer and therefore typically end up costing less over its lifetime of use.

Sealing the Seams

Before ordering a koi pond liner, consider how difficult it will be to seal the seams on the material you select. Even if you’re not handling the installation yourself, you’ll be dealing with leaks if the seams are hard to seal and subsequently become loose over time. For the best results, have a custom liner fabricated with no seams or factory-welded seams where they’re necessary. If the pond liner must be sealed on site, choose RPE or HDPE/LDPE materials since they can all undergo heat-sealed seaming. Hot press tools create tight seals that don’t come loose after a few years underwater, unlike chemically welded seams that can loosen. These adhesive tapes and liquids can also leach harmful chemicals into the water.

Ease of Cleaning

The tear resistance of the liner you choose also determines how easy it will be to clean the pond in the future. All liner materials can eventually grow brittle and run the risk of ripping upon contact with vacuum tubes and nets used for removing sludge. Yet, durable and UV resistant materials last longer than others. Many times, these quality liners boast a lifespan of more than double the average usage period expected from less durable materials. Removing and replacing a ripped or leaking pond liner to is a major task, so choosing liners that won’t be damaged by seasonal cleaning is a much better choice.

With all these tips on koi pond liner selection, you’re ready to make your choice. Explore the entire range of BTL Liners products to choose the right liner for your pond based on your budget and the size of your project.

Liners by BTL

AquaArmor Pond Liner

The most versatile liner on the market today, AquaArmor maximizes protection from harmful UV rays, tear resistance and punctures that cause leaks. Simply the best liner on the market.

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