Preformed garden ponds are available in heavy duty plastic and fibreglass. Due to their limited size, these ponds are only suitable for goldfish and small koi. (Courtesy of Shotgate Koi. © W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

A large formal koi pond. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Pump-fed external filter. Water is pumped from the pond and distributed over the medium through a spray bar. It flows through the medium and returns to the pond, often via a waterfall. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Diagram of a gravity-fed filter. Water from the pond enters the settlement chamber or vortex, where heavy solids settle out. The water passes through a transfer port into the next chamber, where filter brushes trap finer suspended solids. It then passes through a series of chambers containing various types of medium before being pumped back into the pond. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Gravity-fed filter system. Water passes through the circular vortex chamber on the far left then through a chamber of brushes, plastic matting and finally plastic medium before being pumped back into the pond. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Internal undergravel filter. Water is drawn down through the medium (usually gravel) into a pipework matrix and then pumped back into the pond, often through a venturi. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Trickle tower filter. Water is pumped from the pond or settlement chamber, usually passing through a UV lamp. A rotating spray bar distributes the water over the medium. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Fluidized-bed filter. Water pressure keeps the medium in constant suspension. Flow rates are critical to keep the medium suspended and prevent it rising too high in the column. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Bubble-bead filter. Water is pumped under pressure into the chamber. The lightweight beads float to the top, providing a large surface area for bacterial colonization and acting as a mechanical filter by trapping solids. Another design includes heavier beads that are kept in suspension by the water flow and provide additional surface area for biological filtration. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Various types of filter medium: (a) Siporax; (b) Alfagrog; (c) Lytag; (d) Canterbury spar. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Various types of plastic filter medium: (a) Springflo; (b) Flocor; (c) filter brush; (d) Japanese filter matting; (e) foam sheet. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association

Common varieties of fancy goldfish: (a) common goldfish; (b) comet; (c) fantail goldfish; (d) shubunkin or calico goldfish; (e) black moor; (f) oranda; (g) lionhead goldfish; (h) bubble eye. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Other species commonly kept in coldwater ponds: (a) koi; (b) orfe (golden ide); (c) common carp; (d) mirror carp; (e) leather carp; (f) tench; (g) rudd. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2001 British Small Animal Veterinary Association

Common varieties of koi. Koi are classified into groups according to their colour patterns. These have complex Japanese names and the reader is referred to standard texts for a complete account. (Reproduced from The Interpet Encyclopaedia of Koi with the kind permission of Interpet Publishing.) There are four common types of scale pattern in koi:

• Scaled (typical regular carp scale formation)
• Doitsu (scales along dorsum and occasionally along lateral line)
• Leather (no visible scales except for very small ones along the dorsum)
• Gin rin (a gold or silver mirror-like effect on individual scales).

Matsuba is a description of the scale pattern and relates to a darkening in the centre of the scales that produces a ‘pine-cone’ appearance on a single-coloured body. ‘Ghost koi’ are hybrid fish bred from a female mirror carp and male sanke. They vary in colour from dark green-brown like a common carp to a light silver colour, particularly around the head. Grass carp are a different species with olive green or dark green body and small scales. © 2001 British Small Animal Veterinary Association

Mixed varieties of koi. (© W.H. Wildgoose.) © 2001 British Small Animal Veterinary Association