Koi Pond Construction: Essential Requirements & Material Guide

Understanding Koi: Not Just Big Goldfish

Before building a koi pool, understand these remarkable fish. Though related to goldfish, their lifestyle is completely different. Goldfish glide serenely under waterlilies; koi thrash and create havoc. They have specific environmental and nutritional needs—highly sophisticated, inbred fish that aren't easily satisfied.

Environmental requirements dictate construction. A koi pond needs filtered, constantly recirculated water with stable pH, very low ammonia, nitrite, and nitrate, and oxygen near saturation. This demands a continuously running filtration system. For real success, the pond should be at least 1.5 m (5 ft) deep and hold at least 9,000 litres (2,000 gallons).

Traditional garden pools make poor koi ponds. A specialized structure is needed: a raised pool of concrete blocks with brick facing and waterproof render, or a large plastic storage container within a brick surround for stability. For guaranteed strength, choose a double-skinned wall, tied together on a proper foundation.

Filtration: Scale and Integration

Filtration principles mirror garden pools but on a larger scale. Experienced keepers expect the biological filter area to be at least one-third of the pond area, preferably half. A single chamber is more efficient than divided sections. Modern high-tech filters with unconventional media and flow rates require strict adherence to manufacturer instructions.

Most koi owners prefer in-ground, gravity-fed filters built at pond level with large bottom-drain pipes and the pump at the system's end. DIY systems seem economical but are folly for beginners—even with expert guidance, achieving the correct setup without experience is difficult. A manufactured system of correct size, integrated from the start, is most satisfactory and cost-effective.

Choosing Construction Materials for Koi Pools

Liner Options

• Polythene liner: Only for larger excavations sandwiched between soil layers; low burst resistance; no elasticity; perishes in sunlight; difficult to repair.
• PVC liner: Flexible, easy to install; range of colours; economical; 10–15 year life; repairable.
• LDPE liner: Flexible, easy to install; economical; long life; repairable.
• Butyl rubber liner: Flexible, very durable, unobtrusive finish; expensive but easily repaired.

Pre-formed Pools

• Polythene pre-formed: Sunlight resistant; limited shapes; poor plant accommodation.
• Plastic pre-formed: Cheap, sunlight resistant; harder to install; limited shapes; poor plant accommodation.
• Fibreglass pre-formed: Sunlight resistant; most expensive pre-formed but very durable; limited shapes; poor plant accommodation.

Other Materials

• Concrete: Versatile for varied features; needs skilled installation; difficult to repair but very durable if done right.
• Clay (including bentonite): Difficult to install; demands skill to maintain.
• Bentonite clay blanket: Excellent for natural ponds, especially large features; skilled installation needed; self-healing if punctured.