From picking suitable species to ensuring your pond is safe, Hannah Stephenson runs through some key points to consider before you start.

WINTER has been hanging around longer than normal but, hopefully, the ice on your pond should now have melted, so it may soon be time to think about stocking it with suitable fish.

The best time to buy them is when the water temperature is rising and has reached about 10C (50F) - but which are the best buys?

Here's what you need to know...

Choose a suitable fish: Among the most popular, exotic and expensive are koi carp, which are classified by scale types, patterns and colours. But be warned, if you want to keep koi well, your pond must be large and deep, with crystal clear water. Koi and other carp can grow to 75cm or more and some are worth thousands of pounds, so you may want to consider how secure the pond is before splashing out.

A less adventurous option is the common goldfish, which is easily bred and generally no longer than 15cm, ideal for most ponds, and will tolerate water as warm as 35C (95F), although not for long periods.

There are also fancy types of goldfish with enlarged heads and big, bubble-eyes, but many don't tolerate extremes in temperature, such as freezing weather or prolonged periods of heat.

Orfe, rudd and roach are also popular choices, although as many spend a lot of their time near the surface, you may need to protect your pond with netting to stop cats and herons from taking them.

Go to a reputable supplier who will allow you to examine and pick out your fish. The healthiest ones should have bright eyes and a sturdy body, with a stout dorsel fin. Fish with missing or damaged scales are best avoided, as the exposed tissue beneath the scales may be prone to fungal infection. But do remember not to mix exotic with native species, or you'll have a fight on your hands. Sticklebacks, for instance, will eat every goldfish egg as it's laid.

Keep your pond healthy: Whichever fish you choose, the general rule for keeping them healthy is to have well-oxygenated water, varying depths, and sufficient food, shade and shelter in your pond. Get plants established and make sure the water is clear before you introduce the fish. A well-planted pond should ensure goldfish can acquire most of the food they need, although some additional feeding will be necessary.

If you have a wildlife pond, be aware that this is likely to attract other creatures, including predators such as herons and kingfishers, so the fish you choose should match the environment.

In this instance, it may be wise to avoid highly coloured ornamental varieties and go for native species which are cheaper to replace, such as dace, roach and rudd.

Remember that fish can be fussy: Some fish don't like clear water - carp will disturb mud at the bottom of a pond to make it murky and camouflage them. And when buying fish, remember that they feed at different water levels. Koi feed at a minimum depth of 3ft (1m), so avoid shallow pools, while orfe are active surface-feeders, which can grow to 45cm or larger and are unsuitable for small ponds.

Don't overfill the pond: The number of fish you have in your pond depends on its size. Surface area determines how much oxygen is available to them, so work out how many fish you can accommodate by following this general rule: multiply the average length of the pond by its average width, to give you the surface area.

Apply a general rule of 1in of fish to each square foot - that's 25cm of fish per square metre. If in doubt, take advice from reputable pond and aquatic centres. It's better to have too few fish than too many.

Keep the water oxygenated: Deep water ponds also retain a more regular temperature and more dissolved oxygen during warm weather. If your fish dart about, it may be that they have become stressed because your pond is too shallow or there are not enough corners for them to hide.

You can keep fish in a pond without plants, but most gardeners prefer to add some aquatic greenery to make the area seem more natural. And don't worry about algae, because this provides a vital breeding ground for insects on which fish can feed.

Fish should be fed around twice a day in summer, but don't overfeed them. In winter the feeding can be cut down. And in hot weather, top up the pool with a hosepipe or turn on the pool pump to circulate the water more efficiently and create more oxygen.

Acclimatise the fish: To acclimatise fish to a pond, it's best to buy them small. They are generally transported from aquatic centres in plastic bags. To help them adapt to the lower temperature of the pond, sink the unopened bag into the pond for an hour or so, adding a small amount of pond water, then you can gently open it to release the fish.