Stocking a Garden Pond: How Many Koi and Goldfish

Stocking rules of thumb from extension and veterinary sources, growth of koi and goldfish, oxygen and ammonia limits, why overstocked ponds fail, and which species can share a pond.

Why stocking density matters

The Merck Veterinary Manual names low dissolved oxygen and high ammonia as the two water-quality parameters most likely to kill fish directly, and both are driven by how many fish a pond holds and how much they are fed. Practical Fishkeeping lists the variables behind a pond's real carrying capacity: filter surface area, water flow, oxygen level, the body mass of the fish and the amount of food offered. Tropical Fish Hobbyist identifies overstocking and overfeeding, especially while the biological filter is still maturing, as the most common cause of water-quality problems in koi ponds.

Rules of thumb from the literature

Practical Fishkeeping: 10 inches of fish per 100 gallons, or 25 cm per 455 litres, calculated on adult length.
Merck Veterinary Manual (freshwater systems): 1 inch of fish per 12 square inches of surface area, or 1 cm per 36 cm²; the ratio should be reduced for cold-water ponds because beneficial bacteria work less effectively at low temperature.
Tropical Fish Hobbyist: at least 15 US gallons (57 L) per adult goldfish with good aeration; 200–300 gallons (760–1,140 L) per koi, with a 1,000-gallon (3,800 L) pond suited to three or four koi. A second TFH article calls 1,000 gallons the bare minimum for a koi pond and limits it to no more than three adult koi, noting that the often-quoted 300 gallons per koi leaves large fish too little swimming room.
UF/IFAS Gardening Solutions: a fish pond should hold at least 500 gallons (1,900 L); koi need a depth of at least 3 feet (0.9 m), and some keepers recommend 6 feet (1.8 m).
Practical Fishkeeping: 500 litres, with a filter, is the minimum for goldfish outdoors long term in the UK; a 204-litre pond is too small for koi or goldfish.
Tropical Fish Hobbyist: ponds less than 4 feet (1.2 m) deep should house goldfish rather than koi.

How large koi and goldfish become

Koi are a domesticated form of carp (Cyprinus carpio; Wikipedia also gives Cyprinus rubrofuscus). Tropical Fish Hobbyist gives a maximum length of 120 cm (4 ft) for the species and reports koi of up to 3 feet (90 cm) and 40 pounds (18 kg) in ponds. Growth is fastest early: 6–7 inches a year in the first years, slowing to 4–5 inches in the third year, with maximum length reached at 10 to 13 years. Koi live for decades, and the same source describes the species as tolerating 3–32 °C, while Wikipedia gives 15–25 °C as the range in which koi do best. Goldfish (Carassius auratus) typically reach 12–22 cm and can exceed 40 cm (Wikipedia); TFH gives up to a foot for pond goldfish, and lifespans beyond 10 years are common. Both species are heavy waste producers (TFH; Wikipedia).

Oxygen budget

Fish take oxygen from the water through their gills, and warm water holds less of it. UF/IFAS gives saturation values of 11.9 mg/L at 45 °F (7 °C) but only 7.4 mg/L at 90 °F (32 °C). The same source recommends 5 mg/L or more for good fish health, reports distress in most species at 2–4 mg/L and mortality below 2 mg/L; Merck calls concentrations below 5 mg/L dangerous, with deaths possible at 4 mg/L or less depending on species and exposure time. At night photosynthesis stops while respiration by fish, plants and algae continues, so oxygen falls towards dawn, and decomposing waste on the bottom consumes oxygen around the clock. Every extra fish and every gram of uneaten food adds to that demand, which is why UF/IFAS and Merck list overstocking, overfeeding and algae die-offs among the common causes of oxygen crashes. A pump or aerator is essential for a fish pond, and Practical Fishkeeping notes that a larger surface area relative to volume improves gas exchange.

Waste load and the nitrogen cycle

Fish excrete ammonia mainly through the gills, and UF/IFAS states plainly that the more a fish is fed, the more ammonia it produces. Un-ionised ammonia (NH3) is about 100 times more toxic than the ionised form, and its share rises with pH and temperature. UF/IFAS gives 0.05 mg/L NH3 as enough to damage gill tissue and 2.0 mg/L as lethal to sensitive fish; nitrite becomes toxic from about 0.10 mg/L and, according to Merck, forms methaemoglobin in the blood so that fish suffocate even in oxygen-rich water. Bacteria in the filter convert ammonia to nitrite and nitrate, but a new biofilter needs six to eight weeks to mature, so stocking should be gradual. Nitrate accumulates faster with more fish or higher-protein food (Practical Fishkeeping); Tropical Fish Hobbyist advises keeping nitrate at 20–30 ppm (40 ppm maximum) or below 50 ppm in koi ponds, using partial water changes of no more than 50 %.

Why overstocking fails

Ammonia and nitrite spikes that can persist indefinitely when the filter cannot process the extra waste (Practical Fishkeeping).
Oxygen crashes at night and in hot weather, when saturation is lowest and demand highest (UF/IFAS).
Algae blooms fed by the nutrient load of too many fish; uneaten food is itself a sign of a management problem (Tropical Fish Hobbyist).
Reduced growth and survival: a peer-reviewed koi review links higher densities to competition for food and spatial stress.
A smaller margin of error: TFH notes that a lightly stocked pond can absorb the environmental fluctuations to which outdoor systems are exposed.

Mixing species

Koi and goldfish generally coexist peacefully in a large pond (Tropical Fish Hobbyist), and Merck reports that aggression among pond fish such as koi and comet goldfish is rare. The two can interbreed, but Wikipedia notes their offspring are sterile because they derive from different carp species. Wikipedia also lists rudd, tench and orfe as compatible with pond goldfish. Koi are social, and TFH recommends keeping at least three to five. Fancy goldfish varieties are less hardy: Wikipedia states that temperatures under about 10 °C are dangerous to them, so they are not year-round pond fish in cold climates. For very small ponds Practical Fishkeeping suggests White Cloud Mountain minnows for summer only, once the water is reliably 15 °C or above, or keeping the pond as a wildlife pond without fish.