Ponds have to be watertight and, within reason, no matter how you achieve this. Today you'll find three main options:
Prefabricated ponds
Rigid moulded liners created from vacuum-formed plastic will be the cheapest and they also usually feature built-in shelves. They are not too difficult to install, and plenty of people like them as the shape is predetermined, but you are fairly short¬lived. PVC-based and rubberized compounds can be obtained; they're slightly more costly, however are longer lasting. Moulded fibreglass pre-formed liners include the longest lasting of, but they are not really easy to find, and therefore are more expensive if you do.


Flexible liners
These are simply lengths of waterproof sheeting, making it possible to build a pond associated with a shape or size. More planning becomes necessary, and calculating how much liner you will need is not always easy. Lined ponds are perfect for informal schemes, ever since the sheeting will fit most shapes and contours, albeit with varying levels of creasing. Many raised ponds, which to start with appear being constructed entirely of bricks and cement, are in reality lined inside. The best reliability incorporates rubber sheeting generally known as butyl, but PVC and LDPE (low-density polyethylene) sheets will also be to be recommended. Buy lengths which has a guarantee of 2 decades or more. Polythene is typical at the cheaper end in the scale, however it lacks pliability and becomes brittle after prolonged contact sunlight, same goes with best avoided.

Finally you will find 'geotextile' liners, which can be rubber-based liners impregnated with sodium bentonite. These are self-healing liners -if they sustain the puncture, the bentonite will plug the opening.
Concrete ponds
Concrete was the primary choice years back, but has stopped being favoured because creating a satisfactory concrete pond has a great deal of skill, a serious amounts of hard labour. Achieving the right mix, employing it correctly and keeping it workable, are tasks that a great many beginners get badly wrong. Yet, properly designed and constructed, a concrete pond might be elegant and still have an air of permanence unequalled by other materials.
Safety have to be paramount continually, every water feature is often a potential hazard. If a pond is very large enough for plants and fish, odds are it is also large enough for a child to fall in. So safety has to be a major consideration if kids are likely to become present. Choose a site with all-round visibility, so as to keep an eye on children when they're playing in the vicinity.
Barrier fencing is surely an obvious provision, and yes it need not be unsightly. Picket fencing is undoubtedly an effective barrier (which enables it to look quite attractive, too) however it must be no less than 75cm (30in) high. If end posts are slotted into sockets at walk-out it doesn't have to be a permanent fixture. Both metal hoop fencing as employed in parks, and chain link fencing as used around school playgrounds, will merge to the background greenery and become less obtrusive if painted dark green. Heavy grilles placed above the pond might save lighter children from choosing a dip, but it is hardly a nice-looking element for just a garden pool.
The barriers stated earlier will also protect your fish from cats and herons - both the main enemies with the outdoor fish keeper. But as far as students are concerned, to become honest, if the pond poses an enduring risk directly to them, it is better being without it until they reach an age if you do not have to watch them constantly.