Fitting and filling the liner 

Before you fit and fill the liner it is wise to decide how you wish to finish the edge and where you want the water level to be.  If you simply anchor the liner down with paving slabs, the water will never reach higher than the bottom of the slabs (as in diagram z-i) and when water is lost by evaporation (sun, wind, etc.) it will be even lower, exposing the liner to view and to the damaging effect of sunlight.  This can be avoided by leaving enough liner to reach up behind the slabs to ground level.  The water level will then be able to reach as high as the top of the slabs (as in diagram z-ii).The same applies if you use turf instead of paving.  Either way, you need to cut a trench the correct depth for the edging as you will find it very messy to do this after the liner is filled with water.

Once the hole and the edge are correctly shaped and padded, drape the liner across the hole.  This will be much easier if you pick a sunny day and lay the liner in the sun until it is warm and flexible (and if you do it with a friend).   If you are using butyl it only needs to sag loosely into the hole because it will stretch into the nooks and crannies as the weight of the water lying on it increases.   Anchor the edges of the liner with bricks or other objects heavy enough to keep it taut and prevent it  falling into the hole.  Put your hose onto the liner and keep watch as the water moulds it to the shape of the hole.  You will need to move the weights around to allow the liner to creep where it needs to. Once the pond is nearly full, turn off the water and trim off surplus liner, being careful to leave enough to complete whatever edging you have planned.     

Living with your pond

Once you have decided on and created your first pond, the next priority is keeping the water transparent and providing vegetation for decoration and to feed, oxygenate and hide the wildlife.  This is all achieved by the correct balance of plants.

My first pond was filled in early spring to receive the spawn offered in the newspaper.  It soon dawned on me that it would be quite a few years before the frogs resulting from this spawn were big enough to eat the gigantic snails already dominating my garden.  But I was just in time to find two pairs of frogs and one pair of toads all purchased in amplexus¹ (this is no longer legal) soon  to spawn. A few water plants were popped onto the shelf, some bunches of oxygenators were submerged and by early summer I had bought a few fish and the water was like thick pea-soup!  This happens to all new ponds and is very frustrating because you can no longer see the tadpoles, fish or the interesting insects that rapidly arrive from nowhere to live in your pond. The green soup consists of tiny algae floating in the water and these algae will colonise any water left standing in light conditions.  If you do not believe this, leave a clear glass of tap-water on a sunny window-sill for a few days.

Green Pond-water

Try very hard to be patient with green water.  I was impatient and treated the first pond with algicide which did indeed kill the green soup but a few weeks later it was back with a vengeance.  The original algae were killed by the algicide and sank to the bottom as debris.  Organic debris is food and any surviving green algae now had a much improved supply of food to produce the next even stronger generation of pea-soup.  The secret is patience and the correct balance of plants.  Like any other green plant, algae need light and food (nutrients).  They can be deprived of light by leaves that spread out over the surface, and growing water lilies is a very attractive way of doing this. The lilies and any other growing plants use up some of the food, and submerged plants (often called oxygenators) provide further competition for the food.  Even blanket-weed, another type of alga, is a good oxygenator and nutrient consumer.

Another cause of green water is the presence of fish, especially if you feed them, because their excretions increase the stock of nutrients available for unwanted algae.   If you do not feed them, they tend to eat anything they find including tadpoles, dragonfly nymphs, and the early stages of other insects.  This is mentioned again under "Fish or no fish"..

Once algicide had proved an expensive mistake in the first pond, I bought a water-lily (see Pond-plants) and some untreated cork tiles which I floated on the surface to provide shade while the lily leaves grew.  This worked well and I have never used algicide again. When I filled pond 2 for the first time I was ready for the pea-soup with my collection of cork tiles, water-lilies and a new trick.  I had a pair of worn-out bed-sheets which were dark green (all the rage in the 1960s), and I draped one of these across one end of the new pond so that with the cork-tiles, about 60% of the surface was covered.  This combination worked so well that pond number 2 never developed the algal pea-soup. By its second summer, the water-lilies had established enough to manage without cork or sheet.

Pond-plants

Water-lilies and other water plants are ridiculously expensive to buy from shops.  Most people who have ponds, have water-lilies and other plants that grow too big quite quickly so, before spending a lot of money, get to know which neighbours have ponds and offer to take surplus off their hands at pruning time (throughout the summer for most plants). Once your pond is established, you may want to be a bit more selective about the size and colour of your lilies (I am not crazy about those huge pink ones that look like plastic replicas) but while you are getting the water sorted out, a lot of free lily-pads are ideal.

PLANTS FOR UK PONDS

Plants for the bottom (75 cm deep):  Water lilies, Water Hawthorn (Aponogeton distachyus), oxygenators (see below).

Plants for the shelf:  Flowering Rush (Butomus umbellatus)Bog Arum (Calla palustris), Marsh Marigold (Caltha palustris), Bog Bean (Menyanthes trifoliata), Fringed Water-lily (Nymphoides peltata), Monkey Flower (Mimulus guttatus), Pickerel Weed (Pontederia cordata), Arrowhead (Sagittaria sagittifolia), Reed Mace (often incorrectly called Bulrush) (avoid the bigger ones and go for Typha minima), Brooklime (Veronica beccabunga), Water Mint (Mentha aquatica), Irises, Crowfoots and Spearworts.

Floating plants:  Water Fern (Azolla filiculoides).

Oxygenators: Rigid Hornwort (Ceratophyllum demersum); the very popular Canadian Waterweed (Elodea canadensis) is an alien.

Plants not recommended for UK Ponds

Aliens (in UK):  New Zealand Pigmyweed (Crassula helmsii), Parrot's-feather (Myriophyllum aquaticum), 

In the plant table (Plants for UK Ponds) there are lists of plants indigenous to the UK that thrive in different parts of the pond.  You will find many other enticing and exotic ones on sale at nurseries but be wary of things that will not survive local weather and of "aliens".   I am using this word to refer to plants that are imported from outside a country, which if they escape into the countryside will compete with and damage or destroy local species. Apart from the fact that this harms biodiversity (e.g., local larvae will not be able to eat the leaves), there are now a few which it is illegal to introduce into the wild: in the UK Parrot's-feather, New Zealand Pygmyweed, Japanese Knotweed and Himalayan Balsam.  It is better for your own local biodiversity to stick to locally successful plants.

Fish or no fish

A few pretty goldfish can bring life and grace to your pond, watching them grow and learn to come for food can be fascinating, and I had always enjoyed having fish in my ponds. However, there is a price to pay and I proved this to myself with my third pond.  When I was obliged (by leaking) to empty it, I had been wondering for 2 years why there were so few dragonflies emerging when I had seen with my own eyes many Southern Hawkers,  Emperors and other species laying eggs in and near the pond.   I had acquired half a dozen goldfish, four Golden Orfe and a couple of Green Tench.   They all seemed to be thriving and growing and I thought that I was protecting the wildlife by supplying them with bought fish-food .

During the clearing of pond 3, every single bucketful of water was scrutinised and sieved in order to save and replace such things as dragonfly and other nymphs, water fleas, boatmen, back-swimmers, newts, etc.  There were very small numbers of everything and in particular only 3 dragonfly nymphs.  I decided that the fish must be the culprits, simply eating everything that moved.  A neighbour with a much bigger pond was happy to adopt them, otherwise I would have asked a nursery to take them,  as exotic ("alien") fish should never be introduced into wild ponds, lakes or rivers.  Four years later the water was seething with life and I was able to watch literally dozens of Southern Hawker dragonflies emerging and flying away, so I think the fish were indeed causing massive depredation. 

A final thought about excluding fish. The presence of fish will attract the attention of several predators including herons, gulls, kingfishers, mink and otters (and, of course, domestic cats). As most of these will also eat frogs and newts, you just have to make your own decision about who gets  priority.