Koi that rush the pond edge when you appear are performing a learned routine: you are the most reliable event in their world, and the edge is where your food lands. So when those fish begin holding deep, hanging under the lily pads and refusing to come up, treat it as information. Koi do not abandon the greeting habit out of mood — something in their environment has re-priced the risk of the surface, and your job is to work out what, in the order of likelihood.
First suspect: a predator has visited
The commonest cause, by far, is a scare. A heron standing in the shallows at dawn, a cat fishing from the edge, even a neighbour's dog crashing into the pond can mark the group, and koi remember: survivors of a heron strike might stay deep and skittish for a week or more, and a missing or injured fish confirms it. Walk the pond edge looking for evidence — a heron's footprints in soft margins, disturbed netting, feathers, displaced stones — and check whether the fish return to the edge at dusk or only while you are absent, which suggests they now associate the surface itself, not you, with danger.
Second: test the water before anything else
Behaviour is the pond's earliest water-quality alarm. Warm, still weather drops oxygen overnight, and koi respond by hanging in the deepest, coolest water and refusing food — check early in the morning, and look for any fish piping at the surface. Test ammonia and nitrite, particularly if you have recently increased feeding, added fish, or the filter has been off or cleaned hard; rising waste makes fish sullen and edge-shy long before it makes them visibly ill. Note the temperature: a sharp cold snap sends koi deep and off their food normally for a day or two.
Third: watch the fish themselves, from a distance
Because your approach is part of the problem, observe from a window or a chair well back. Read the group: are they schooling tightly at depth (a fear pattern), or scattered and still (more like water or illness)? Look for flashing — fish flicking their sides against the pond floor — clamped fins, one fish isolating from the group, or excess mucus giving skin a cloudy look. A single isolating, flashing fish alongside a cautious pond suggests a parasite irritation troubling the group, while uniform spooky behaviour with clean flick-free swimming keeps the heron at the top of the list.
Fourth: audit what else has changed
Work through the last week honestly. New fish added without quarantine can import both disease and a hierarchy that unsettles feeders. Treatments, a big water change, building work vibrating the ground, a new overhanging branch or a moved statue casting a heron-shaped shadow over the shallows — koi respond to silhouettes, and a changed shadow can pin them down as effectively as a real visit. Even a change in your own routine, approaching from a new direction or bringing a stranger to the edge, can suppress greeting in a pond that has had a scare.
When it stops being detective work
Get a fish vet or pond health specialist involved if you find ulcers, sores or raised scales, if fish are gasping at the surface or clustering at the filter return, if flashing is constant, or if any fish dies — those move the situation from behavioural to medical, and pond problems escalate fast in warm water. When you call, the history is half the diagnosis: the date the greeting stopped, your morning and evening readings since, what you suspect about a predator. Keeping those test results and vet notes together in the koi's Pet Passport on KinsLe means the story travels with the fish, ready for a visit rather than reconstructed from memory.