Garden Pond Basics for Goldfish and Koi
Siting and Depth
Decide which fish you want first, because koi need a much bigger, deeper pond than goldfish. Then choose a level spot that gets about 4–6 hours of sun a day with some shade, away from overhanging trees, and with a safe outdoor power supply for the pump.
Depth gives fish a stable zone: cooler in heat waves and warmer at the bottom when the surface freezes. OATA suggests at least 18–24 inches (45–60 cm) for fish ponds and over 3 feet (90 cm) for koi. In very cold climates, check local advice on how deep ponds need to be so they don't freeze solid.
Liner or Preformed?
- Preformed ponds are rigid shells of set sizes. Choose one at least 2 feet (60 cm) deep in part, with shelves for plants, and bed it level on sand.
- Liners let you make almost any shape. Dig shelves and a deep zone, remove sharp stones, lay underlay, and let the water's weight settle the liner.
Keep fresh mortar out of the water, because it can make it dangerously alkaline for fish. Dechlorinate tap water, run the pump and filter for at least a day to check for leaks, and let plants and the filter mature before adding fish, testing the water as you would a new aquarium.
Pond Equipment
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A pond holds more water and more fish than any tank, so use a pond pump and filter rated for your pond's volume. A pond test kit tracks water quality, heron netting held above the surface protects fish, and a rigid safety grille makes the pond safe around young children.
Predators
Herons, raccoons, cats and in some places otters take pond fish. OATA suggests netting held about 12 inches (30 cm) above the water to deter herons, and heavy-gauge mesh rather than ordinary netting where otters visit in winter. Deep areas and plants give fish places to hide.
Many predators, including herons and otters, are protected by law in many countries, so deter them; never harm them. In the UK, for example, it is an offense to disturb an otter's resting place. Check your national wildlife rules.
Which Fish Belong Outdoors
Common goldfish, comets, shubunkins and sarasas are hardy pond fish. OATA says pond goldfish commonly reach up to about 12 inches (30 cm) and can live 20 years. Fancy goldfish are more delicate and belong indoors; see goldfish tanks done right.
Koi can grow to about 30 inches (75 cm), need a larger, deeper pond and stable, good-quality water. Plan the pond for their adult size from the start.
Never release pond fish or pond plants into the wild. Many are invasive, and releasing them is illegal in many places.
Frequently Asked Questions
What people ask before the first spade goes in.
How deep should a fish pond be?
OATA suggests at least 18–24 inches (45–60 cm) in part, and over 3 feet (90 cm) for koi. In very cold climates, go deeper so it can't freeze solid.
Is a pond safe with toddlers?
Not without protection. RoSPA advises a rigid grille, a secure fence at least 1.1 meters (about 3.5 feet) high with a locked gate, or filling the pond in until children are over six.
Does a pond need a filter?
A pond with fish does. Use a pond filter and pump rated for the pond's volume, not aquarium equipment.
Can fancy goldfish live in a pond?
Generally not. They are slower, more delicate and easier prey, so they are better indoors.
Do koi need a different pond?
Yes: bigger, deeper and with stronger filtration, because koi can reach about 30 inches (75 cm).
How do I stop herons taking my fish?
Netting held above the water works best, along with deep areas and plants for cover. Herons are protected in many countries, so deter them; don't harm them.
Related Reading
- Goldfish tanks done right: the indoor option.
- Water care and testing: what the numbers mean.
- Hot weather safety: pets, water and algae.
If pond fish gasp at the surface, stop eating or develop sores, test the water first, then ask trained aquatics staff or a vet who treats fish.