Start with these checks
- Watch the remaining fish. Gasping or sudden distress needs prompt water checks and advice from an aquatic vet or koi specialist.
- Count the fish from more than one position and check hiding places without chasing them or dismantling the pond.
- Look around the edge, overflow and nearby ground for a route a fish could have jumped or escaped through.
- Check pumps and aeration. Record ammonia, nitrite, pH and temperature using suitable tests; check oxygen if you have the equipment.
- Review sightings and footage from when you last saw the missing fish. Note any damaged covers, remains or disturbed equipment.
If you find signs of a visitor
Photograph footprints, droppings or disturbed planting before tidying up. Compare them with any camera footage. A heron can take fish without leaving much behind, but a missing fish on its own does not identify the cause.
Use what you find to check the relevant edge or gap in a cover. Other wildlife may also visit, depending on where you live.
If the remaining fish look unwell
Write down water readings and any changes in behaviour. Gasping, rubbing against surfaces, clamped fins or visible sores are reasons to investigate fish health, even if a heron has also been seen.
Contact an aquatic vet or koi specialist if fish are distressed or losses continue. Have your test results and details of recent feeding, maintenance or new fish ready when you ask for help.
If you still cannot explain it
Keep a short log of fish counts, water readings and visits over the next few days. Check access to the garden and disturbed equipment if you suspect someone has entered.
tallGuard can record a heron in its camera view, which may help establish what visited. A gap in the recordings does not rule out a predator, and the camera cannot tell you whether the water or fish are healthy.