Garden Pond Fish Care

Keeping fish in a garden pond: pond depth and volume, goldfish and koi needs, oxygenation, and seasonal and winter care.

Overview

Garden ponds let coldwater fish such as goldfish and koi be kept outdoors year round in temperate climates. Their large water volume buffers temperature and chemistry changes, but the pond must be deep and well oxygenated enough to keep fish safe through summer heat and winter cold.

Depth and volume

Adequate depth protects fish in winter so the pond does not freeze solid. For koi a minimum depth of about 1 metre is suggested in milder regions and at least 1.5 metres (5 feet) where winters are harsh. A reported minimum for keeping goldfish outdoors long term is around 500 litres with filtration, and small shallow ponds of roughly 30 cm deep risk freezing through.

Goldfish and koi

Goldfish are hardy coldwater pond fish, while koi (Cyprinus rubrofuscus) grow larger and need more space and depth. Koi do best at around 15-25 °C (59-77 °F); their immune systems become very weak below 10 °C (50 °F), so warm, stable summers and adequate winter depth both matter.

Oxygenation

Oxygen is vital for both fish and the filter bacteria, so a well-oxygenated pond is healthier. Turning the water over with a pump and fountain or a dedicated air pump raises oxygen levels. Because oxygen demand peaks at night, aeration should not be switched off overnight in the warmer months.

Winter care

In cold water, koi digestion slows almost to a halt and feeding is not recommended once the water falls below about 10 °C (50 °F). An ice sheet traps gases, so a hole should be kept open in the ice, for example by melting it with a saucepan of hot water or a floating pond heater, rather than smashing the ice and shocking the fish.

Seasonal aeration

Cold water holds more oxygen than warm water and the fish need less of it in winter, so fountains and aerators are often turned off in the coldest months. In summer the opposite applies, and strong aeration helps avoid oxygen shortages during warm nights.