On this page
Overstocking is the most common cause of green water, poor water quality and sick fish in garden ponds. Fish also grow, and goldfish breed. This guide gives you simple rules for working out how many fish your pond can safely hold.
Step 1: Work out your pond's volume
For a rectangular pond: length x width x average depth (in metres) x 1,000 = litres.
For a round pond: 3.14 x radius x radius x average depth (in metres) x 1,000 = litres.
For an irregular pond, measure the average length and width, multiply by the average depth, and multiply by 1,000. If you're unsure, underestimate.
Step 2: Use a stocking rule
These are guidelines for a pond with good filtration. Measure fish from nose to the base of the tail, and use their adult size.
| Fish | Rule of thumb | Minimum pond |
|---|---|---|
| Goldfish, shubunkins, comets | About 2.5cm of fish per 25–50 litres | 500 litres, 60cm+ deep |
| Koi | About 1 adult koi per 1,000 litres | 4,500 litres+, at least 1.2m deep |
| Wildlife pond | No fish | Any size |
For example, a 2,000-litre pond could hold around eight to ten goldfish, allowing for growth. That same pond is too small for koi.
Surface area matters too
Oxygen enters through the surface, so a wide, shallow pond can hold more fish than a narrow, deep one of the same volume. A waterfall, fountain or air pump increases oxygen and helps in summer.
Filtration
A good filter can let you keep more fish, but there's still a limit. Choose a filter rated for at least your pond volume, and for koi, a filter rated for double. See pond filters and our complete guide to pond filters.
Signs your pond is overstocked
- Green water or blanketweed that won't clear. See Green Pond Water and Blanketweed.
- Fish gasping at the surface, especially early in the morning.
- High ammonia or nitrite readings. Check with test kits.
- Frequent disease. See Common Pond Fish Diseases.
Adding new fish safely
- Add fish to a new pond gradually, a few at a time over several weeks, to let the filter catch up.
- Float the bag for 20–30 minutes so the temperatures match, then slowly add pond water.
- Quarantine new fish where possible.
- Avoid adding fish in winter when the filter is less active. Spring is the best time.
What to do if your pond is overstocked
Rehome some fish, upgrade the filter, feed less and increase aeration. Goldfish breed every year, so thin out young fish before numbers get out of hand.
New to koi? Read Koi Care for Beginners.
