The short answer for a kuhli loach
A healthy kuhli loach is invisible by day and busy by night. It drapes itself under wood, threads through plants after lights-out, and snuffles through the substrate with its whiskery barbels hunting sunken food. A kuhli loach in pain or discomfort breaks the night half of that pattern: it stays wedged in its hiding spot through its active hours, ignores food it would normally find within minutes, or appears in daylight behaving restlessly, looping up the glass. With this species, you diagnose change by schedule, not by a daytime glance.
The nocturnal baseline of an eel-like bottom fish
In a soft-substrate tank with plenty of cover, a settled kuhli loach spends daylight hours tucked under leaf litter, wood, or stone, often with several loaches piled companionably in the same retreat. As light fades they emerge, gliding in smooth eel-like curves, sifting mouthfuls of sand, and investigating every corner for morsels. Bands stay rich brown against warm tan, the belly looks gently rounded, and the barbels around the mouth stay intact and active. A loach that keeps that dusk appointment night after night is almost certainly comfortable, however invisible it seems at midday.
How discomfort surfaces in a hiding loach
In a kuhli loach, the key warning is a missed emergence. If the loach that always appears when food lands after dark stays hidden across two evenings, something has changed. When it does show, look for faded banding, a pinched thin body, and barbels that look short, white-tipped, or eroded, which can follow frantic digging in rough substrate or poor bottom conditions. Daylight restlessness is another inversion to respect: a loach racing the glass at noon is not being playful, it is behaving like a fish trying to escape its circumstances. Lying exposed on open sand, breathing fast, is further still from this species' instincts.
Night checks that respect how this loach lives
For a kuhli loach, do your detective work after lights-out with a dim room lamp rather than lifting its shelter at noon. Drop a sinking morsel near its known retreat and watch whether barbels start working and the loach slides out to feed. Sift your attention to the bottom layer: test the water, check for compacted or fouled substrate under decor, and confirm hiding places have not been rearranged or claimed by another bottom-dweller. Count the loach group if you keep several; a whole group missing a dusk feeding points at the tank, not one fish.
When a kuhli loach needs an aquatic vet
A kuhli loach that has missed several night feedings, shows eroded barbels with redness, visible sores, rapid breathing while exposed, or a body that looks wasted along the back should be assessed by an aquatic veterinarian. Sudden deaths among other bottom-dwellers alongside loach changes raise the urgency further. Bring notes on emergence times, which retreats it uses, substrate changes, and test results, because a vet treating a secretive fish leans heavily on the keeper's night observations. Recording those dusk checks in KinsLe's Pet Passport means the pattern behind one odd evening is already documented when you need it.