All carp, and many other fresh water fishes, belong to the order Cyprinidae. Carp varieties include the well-known koi (Cyprinus carpio) and the goldfish varieties from the common goldfish (Carasius auratus) to all fancy varieties bred from it. These include the shubunkin, pearlscale, oranda, lionhead, black moor, comet, and others. Many of these fish can be kept outdoors in England and other temperate climate areas throughout the winter even when some ice forms on the water. However, the more fancy varieties like pearlscale, oranda, lionhead, and black moor appreciate not being subjected to more than an occasional day of ice coverage.

If the pond has a very deep area, so much the better so the fish can descend to the bottom during severe weather. Some koi keepers recommend a pond as deep as six metres (seven feet) to encourage deep body growth. A friend of mine keeps his koi in a pond of 1.3 metres (four feet) plus, a satisfactory depth.

Most koi enthusiasts in England never use plants (including waterlilies). They maintain their ponds using powerful pumps to provide a constant exchange of water through vast containers filled with a variety of filtering media. Although I know many of you who keep koi with your lilies, I hear of many instances of the waterlilies being seriously disturbed by the koi rooting the plants out of their pots. Goldfish varieties do not reach the size of koi and, although they enjoy rummaging around plants (and eating them), they are not a threat to established lilies. Koi, remember, can reach almost a metre (three feet) long in some circumstances.

The cold-water sturgeon (Acipenser sp.) is becoming popular in English home ponds. The splendid site www.sturgeon-web.co.uk offers all the information you would ever need about these fascinating fish. This supplier and expert of the species is in England, but if you are non-English and wish to acquire sturgeon, I am sure suppliers operate in other countries.

Pondkeepers fortunate enough to reside in a more temperate zone (winters without day-long ice coverage) find that they can keep a variety of tropical and semi-tropical fish out of doors throughout the seasons. This happens in the wild, too. Reportedly, non-indigenous fish species have been introduced into Florida lakes and have become a nuisance.

The Oscar (Astronotus ocellatus), for example, has become well established and, being an aggressive big fish, seriously threatens some of the species natural to the state. Such a fish, when full grown, may well disturb planted lilies.

 I keep many of my favourite fish species in a heated greenhouse pond. These include archer fish (Toxotes jaculatrix), uaru cichlid (Uaru amphiacanthoides), silver dollar (Metynnis) which have bred in the pond, zebra (Danio rerio) which breed frequently, lace gouramies (Trichogaster leeri), cardinal tetras (Paracheirodon axelrodii), plecostomus catfish species (Hypostomus plecostomus), and even discus (Symphysodon aequifasciatus). All of these thrive extremely well.

However, I emphasize that the pond is kept never knowingly lower than 80F (27C) degrees. It does descend to around 75F (24C) degrees and the stock could stand 70F (21C) degrees, but uaru and especially discus would not enjoy too long as low as that. With the exception of lace gouramis and zebras, most of the others require soft acid water (discus and uarus in particular), and for this I use both rainwater and RO (reverse osmosis) water.

The archer fish, although a lover of brackish water, tolerates fresh water. It's great fun to see them fire water at flies and other insects, mostly swimming near the surface of the water. They can jump out of the water and end up as dried fish on the ground.

Only experienced aquarists should consider keeping discus because their water conditions and other needs must be strictly monitored; and they're quite expensive fish to buy.

You can keep many aquarium fish varieties in a pond wherever in the world you find a favorable temperature range. Some varieties can live in a wide variety of water temperatures and are quite hardy. The viviparous swordtail (Xiphophorus helleri), both in the well-known red form and colour variations, are very hardy and fast swimming.

Paradise fish (Macropodus opercularis) sport a gorgeous mixture of reds, blues, and pale yellow. They withstand a very wide rage of temperature from as low as 50F (10C) degrees up to being comfortable at 90F (32C) degrees, and live for up to eight years.