Pros and cons of preformed ponds and liners for keeping koi

If you are going to keep koi, or goldfish for that matter, you're going to need a suitable fish pond. Not only must it have the ability to hold water, but it must also be large enough for your koi fish to grow and thrive. While goldfish will generally flourish in any type of pond, big or small, koi are more expensive; they grow larger; and they need considerably more care.

If you are going to build your own koi pond, your choice may be limited by your construction skills. If so you're going to be looking for relatively simple off-the-shelf solutions. The two most common options are:

1. preformed ponds which may be rigid or semi-rigid, and

2. flexible liners.

The preformed pond

When it comes to installation a preformed rigid pond is the simplest option. All you do is mark out the shape of the pond and dig a hole that will accommodate it. Level the pond, backfill around the edges with soil, fill with water and you'll be ready to introduce your koi or goldfish to their new home. There are lots of different types from which to choose, but on the down side, they are usually relatively small. This means that if you are keeping koi, you're going to have to carefully limit their numbers.

In some countries, including the US, the most common preformed ponds are fibreglass, made by moulding glass-fibre and some kind of resin to the required shape. A good quality pond will last for decades. A variety of thermoplastic materials are used to make semi-rigid preformed ponds. The most common method is vacuum-moulding, which is not as expensive as moulding with glass-fibre – but it won't last as long. Also, while fibreglass ponds are available in a variety of colours including black, green, blue and grey, plastic ponds are usually either dark green or black. Happily, black and green are popular colours because they look more natural in the ground.

Another preformed option is made from glass-fibre reinforced cement. This is a similar material to that used to make some fake rocks and ponds can look very effective.

Some distributors sell preformed ponds with a pump and filter. If you're stocking the pond with koi this is an essential requirement both for the health of the fish and so you can see their gorgeous colours. If you are only planning to keep goldfish, then a filter isn't necessary, and you will only need a pump if you want to include some sort of moving water in or around the fish pond.

The flexible liner

A variety of flexible liners may be used to line both koi ponds and ordinary goldfish ponds. On the plus side, a suitable flexible liner may be used to install ponds of just about any size and shape. On the down side, many of these materials puncture easily and they don't last very long. Another factor to consider is whether single sheets can be easily joined. If not, size will be limited by the width of the sheeting available.

The least expensive liner is made from polyethylene, which is the same material that is used to damp-proof buildings. Various colours and thicknesses (or gauges) are available, although black is best for ponds because it contains carbon that inhibits the negative effects of the sun's harmful ultra-violet (UV) rays. Some companies do manufacture 500 micron polyethylene for use in ponds which is thicker than regular 250 micron roofing or under-floor damp proofing. But there's no getting away from the fact that it can puncture easily. It is also more difficult to join than other flexible liners.

A better, middle price-range flexible option is PVC (polyvinyl chloride) which is also used to line some swimming pools. It is a UV-stabilised product that is suppler than the cheaper plastic.

An even better option is EPDM, an ethylene propylene polymer that was originally produced for roofing. It is often used for installing ponds for koi and goldfish in both the UK and USA.

An even more expensive option is butyl rubber, often called the Rolls Royce of pond liners. Thick, flexible, and easy to join, it works well. However folds cannot be avoided and you will need to camouflage these with rocks and water plants. Like all flexible liners, the upper edge of koi and goldfish ponds made with a butyl rubber liner will need to be finished in a creative way, with rocks, plants or an edging of some sort. While this makes no difference to the koi or other fish you decide to stock, it will make it look either more natural or attractive.

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