Koi will spawn in almost any pond that gets warm enough, whether you planned it or not. The question for most keepers is not how to make it happen but how to manage it — a spawn dumps enormous amounts of milt and egg protein into the water and can crash the pond in a day.
When Koi Spawn
Koi spawn in late spring to early summer, triggered by water warming through 65–75°F (18–24°C) and by lengthening daylight. Fish must be sexually mature — typically three years old and 10–12 inches. Spawning usually happens at dawn, and the whole event lasts a few hours.
The signs are unmistakable: males chase a female relentlessly, nudging her flanks and pushing her into shallow water and plant cover; the water turns cloudy and foamy; and eggs appear as tiny sticky beads on plants, brushes and liner walls.
Conditioning Breeders
- Select healthy adults, at least three years old, with good body shape. Use two or three males per female.
- Raise feeding through spring — high-protein food two or three times daily once water passes 60°F.
- Keep water quality tight; ammonia and nitrite must read 0 ppm before a spawn, because they will spike after one.
- Provide spawning media: spawning brushes, ropes, or dense soft plants such as hornwort and water hyacinth roots.
Koi Fish Eggs
| Detail | Figure |
|---|---|
| Eggs per spawn | Roughly 50,000–100,000 for a large female (about 100,000 per kg of body weight) |
| Egg size | 1–2 mm, sticky, translucent amber |
| Hatching time | 3–7 days; about 4 days at 68°F, faster in warmer water |
| Fertile vs infertile | Fertile eggs stay clear or amber; infertile eggs turn opaque white and fungus within 24–48 hours |
| Survival to adulthood | Very low in a stocked pond — adults eat most eggs and fry |
If you want fry, move the spawning media with eggs attached to a separate tank of pond water within a few hours. Remove white fungused eggs daily; they spread fungus to healthy neighbours. If you do not want fry, leaving the eggs in the pond is usually enough — the adults will eat them.
Managing the Pond After a Spawn
This is the part that catches people out. A heavy spawn puts a large protein load into the water and strips oxygen at the same time.
- Do a 30–50% water change as soon as spawning ends.
- Increase aeration immediately — foam on the surface is a sign that oxygen exchange is being blocked.
- Clean skimmers and mechanical media, which will be loaded with egg debris.
- Test ammonia daily for a week. Post-spawn ammonia spikes are common even in well-filtered ponds.
- Check females for damage. Chasing scrapes scales and tears fins; treat injuries and watch for secondary infection.
- Do not feed heavily for a couple of days while the filter catches up.
Baby Koi Fish: Raising Fry
| Stage | Age | Care |
|---|---|---|
| Hatch | Day 0 | Fry hang on surfaces absorbing the yolk sac; do not feed |
| Free-swimming | Day 2–4 | Start feeding: infusoria, liquid fry food, then newly hatched brine shrimp |
| Early growth | Week 2–6 | Powdered fry food several times daily; gentle sponge filtration only |
| Crumble stage | Month 2–3 | Move to crushed pellets; fry reach 1–2 inches |
| Colouring up | Month 6 – year 3 | Pattern develops gradually; final quality is not clear until year three or four |
Two realities worth knowing before you raise a spawn. First, most fry are brown. Wild carp colouring is dominant, and typically only a small percentage of any spawn shows worthwhile koi pattern. Second, culling is part of the practice — breeders select repeatedly and cannot keep tens of thousands of fish. If you are not prepared to rehome or humanely cull, do not collect the eggs; let the pond handle it.
Fry need a sponge filter rather than a powered intake (which will draw them in), 25–30% water changes several times a week, and generous space — overcrowded fry stunt permanently.
Frequently Asked Questions
Can koi fish get pregnant?
No. Koi are egg scatterers, not livebearers, so there is no pregnant koi. A female swollen in spring is gravid - carrying eggs she will release during spawning. A koi that stays swollen outside spawning season, especially with raised scales, has a health problem such as dropsy.
When do koi fish spawn?
Koi spawn in late spring to early summer, triggered when water warms through 65-75F (18-24C) and daylight lengthens. Spawning usually happens at dawn and is over within a few hours.
How many eggs do koi fish lay?
A large female scatters roughly 50,000 to 100,000 eggs in a single spawn, about 100,000 per kilogram of body weight. Survival to adulthood in a stocked pond is very low because the adults eat most eggs and fry.
How long do koi eggs take to hatch?
Three to seven days depending on temperature - about four days at 68F. Fertile eggs stay clear or amber; infertile ones turn opaque white and fungus within a day or two and should be removed.
What do baby koi fish eat?
Newly hatched fry live on their yolk sac for two to four days, then take infusoria or liquid fry food, followed by newly hatched brine shrimp and powdered fry food. From about two months they move on to crushed pellets.
Why are my baby koi brown?
Wild carp colouring is dominant, so most koi fry are brown or grey. Only a small proportion of any spawn develops worthwhile koi pattern, and final quality is not clear until the fish are three or four years old.