Preformed plastic ponds are often the liner of choice for inexperienced pond owners. The options in this class are limited, so presumably, the decisions about shape, size, and depth are easy to make. Take a quick look, and it's easy to assume that installation should be as simple as dropping the form into the ground and sticking a hose in for a speedy fill. That sounds ideal, but there are good reasons that experienced pond owners are quick to move away from preformed ponds in favor of better options:
- Design. Rigid plastic pond liners are only available in a limited variety of sizes and shapes. Since the plastic is too thin to be self-supporting, sizes are kept relatively small to avoid damage in transit. Typical forms are rectangular, oval, and kidney-shaped, with simple variations in depth. If this is exactly the look you're going for, that's a happy coincidence, but if you prefer a look that will blend naturally into your garden, there are very few options among preformed liners.
- Capacity. Preformed ponds rarely come in depths more than 18 to 24 inches which generally support only a few goldfish. Koi, a favorite for backyard pond enthusiasts, need at least 3 to 4 feet of depth to grow and thrive. Even a typical 6 feet by 4 feet pond at 2 feet deep can only support about 24" of mature fish length (a maximum of 6 fish no longer than 4" at maturity, for example). Keep in mind, too, that the closer you are to the population max, the more work it will require to keep the pond healthy. A pond that's at least 3 feet deep, even if it has the same surface area, will offer endless advantages.
- Installation. Even though manufacturers promote preformed ponds as the simplest "drop-in" solution, the plastic is weak. It requires substantial support on all sides to counteract the outward force of the water, meaning the hole itself must be carefully marked and dug to the right shape and depth, then several inches of sand added to provide firm support. Otherwise, the natural movement of soil through changing seasons and rainfall will lead to cracks and leaks. Moreover, a preformed pond liner with changing depths for plant shelves or other features presents an even more complex installation process. Every change in depth requires a new layer of sand, so the installation may involve multiple steps, including graduated filling and settling time -- a far cry from the idea of a simple drop-in.
- Setup. New pond owners may mistakenly believe that they can dip into the hobby "on the cheap" with the ease of a prefab pond where all the work is done for you. Unfortunately, a pond owner taken in by this misconception will almost certainly be disappointed. Fish are living creatures, after all. They eat, they breathe and they produce waste. Their space gets dirty when leaves and dirt blow in, and they can't do a thing about it. Since a constructed pond is not a natural environment, supporting equipment like a pump, filter, and aeration are still necessary to keep the fish healthy. Skipping this step in the spirit of a bare-bones budget marks the road to disaster.
- Equipment. You'll still have to cut and drill your preformed liner to install necessary filtration features such as pumps, filters, and skimmers. Rigid plastic, however, is difficult to modify, and those that have been modified often develop leaks that are hard to find and nearly impossible to repair. Even those preformed liners manufactured with integrated skimmer openings present additional complications. The choice of equipment that fits into the manufactured configuration is often limited, locking you into a specific model and size, which may be expensive or difficult to locate.
- Temperature. Shallow ponds are vulnerable to rapid temperature changes from day to night and from season to season. The warmer the water is, the less dissolved oxygen it can hold. When a shallow pond can't maintain a moderate temperature throughout a day/night cycle, it can quickly become oxygen-depleted and suffocate the inhabitants, even on a mild sunny day. Tropical fish are often unable to tolerate nighttime temperatures if the water doesn't retain enough heat.
- Protection. The prospect of a fresh fish buffet is nearly irresistible to raccoons, cats, and any number of bird species. Typically, fish prefer to take refuge in deep parts of a pond (where it's also comfortably cool), under floating vegetation, or among the branches of a submerged tree to hide and protect themselves from hungry predators. This kind of protection is nearly impossible to provide in a tiny, shallow pool. You'd be amazed at how quickly the local wildlife notices your new pond. If an egret or heron visits, it is likely to eliminate your fish population in a single night, with such ideal hunting conditions.
- Upgrades. When the pond bug has bitten, it's hard to stop. Nearly all pond owners find themselves dreaming endlessly of expanding and upgrading their version of paradise. Happily, this pursuit of perfection can bring endless seasons of satisfaction. Preformed ponds, though, are unchanging. Even when minor improvements are involved, the preformed pond liner usually must be removed entirely and replaced with a flexible pond liner.
- Maintenance. Even tiny ponds require regular maintenance - in fact, the more fish you have in that small space, the more upkeep is needed. Frequent removal of all leaves and debris, managing filtration, and preventing rainwater or sprinkler overflow from entering your pond is necessary to keep the pond in balance and free of uncontrolled algae growth. A serious algal bloom in a tiny, preformed pond is often a death knell for a new pond owner.
Damage and repairs. Preformed ponds are particularly vulnerable to the scraping of metal tools during maintenance, which weakens the surface and can lead to leaks. The rigid material doesn't flex well, so unevenness in the supporting soil, an unfortunate trip or fall, or even debris blown in by a storm can leave a preformed pond badly damaged, with holes or cracks. In such a case, patches may work, but that involves removing water from the pond, drying the damaged area thoroughly, and perhaps even roughing up the surface with sandpaper (carefully!). In all cases, you must take care to use plant- and fish-safe materials.
