Fish
Loach
Cobitidae and Botiidae · also called loaches
A loach is a whiskered bottom-dweller that sifts through sand and gravel across Eurasia’s rivers and rice paddies. One kind, the weather loach, has been read as a living barometer for centuries: drop the air pressure and it turns restless well before the first thunderclap. Another kind, the clown loach, alarms new aquarium owners by sleeping flat on its side, apparently dead.
Overview
Loaches are slender to eel-like bottom fish of the families Cobitidae and Botiidae, spread across fresh water in Europe, North Africa, and especially Southeast Asia. They sift the substrate with sensitive barbels for worms, larvae, and small invertebrates, and many, like their catfish relatives the corydoras, can gulp air and absorb oxygen through the gut, surviving warm, stagnant water.
The weather-sensing loach
The weather (dojo) loach senses falling barometric pressure through its swim bladder and turns restless and erratic — genuinely more active before a storm, which is why it has long been treated as a living forecaster across Asia, and later farmed as food in Japanese dojo hotpot.
Social sleepers
Clown loaches and other Botia species are strongly social, needing groups of five or more to stay calm and confident, and famously rest lying on their side or upside down, a habit that regularly convinces new keepers a perfectly healthy fish has died.
Status
Least Concern for the widespread farmed and aquarium species, some now invasive outside their native range. Many river-restricted loach species are threatened by dams, pollution, and gravel extraction.
Loach: common questions
How does a weather loach predict storms?
It does not truly predict weather but reacts to physical changes that often precede a storm, chiefly a drop in barometric or atmospheric pressure. The loach's swim bladder, a gas-filled organ normally used for buoyancy, is sensitive to pressure changes, and when pressure falls the fish appears to become uncomfortable or agitated, swimming more erratically, dashing about, or coming closer to the surface than usual. Because falling pressure frequently comes before storms, the fish's restlessness often does line up with approaching bad weather, which is how it earned its reputation and its common names in several languages.
Why does my clown loach lie on its side, and is it dying?
In the great majority of cases this is completely normal resting behaviour and not a sign of illness. Clown loaches, and to a lesser extent some other loaches, often rest lying on their side, propped against a piece of wood or decor, or even flat on the substrate, sometimes so still that new owners are convinced the fish has died. A resting loach will usually respond to a light tap on the glass or the approach of food by righting itself and swimming normally. Genuine illness usually comes with other signs such as clamped fins, laboured breathing, loss of colour, or a lack of response to stimulation.
Do loaches need to be kept in groups?
Many of the popular aquarium loaches do best in groups, and some, especially clown loaches and other Botia species, can become stressed, hide constantly, or turn unexpectedly aggressive toward tankmates if kept alone or in pairs. A group of five or more lets the fish establish a stable social order and generally produces calmer, more confident, more visible fish that display natural behaviours such as play-chasing and communal resting. Not every loach is equally social, and some smaller or more solitary species tolerate being kept singly better, so it is worth checking the needs of the specific species.
Can loaches be kept with other fish?
Most loaches are peaceful toward fish that are not loaches and make good general community fish, provided tankmates are not small enough to be seen as food, since a large loach will eat very small fish, shrimp, and snails. The main considerations are substrate, since many loaches like to dig or need fine, smooth sand or gravel to avoid damaging their barbels, and water flow and oxygen levels, which should be higher than for many other community fish. Loaches are also efficient snail eaters and are sometimes deliberately added to a tank to control a snail population that has grown out of hand.