Why a kuhli loach needs choosing for, not just adding to
A kuhli loach is a slender, eel-like bottom-dweller that spends daylight hours tucked under wood, leaves, and plant cover, emerging at dusk to sift the substrate for food. It has no true scales, so its skin is easily damaged, and it has no way to defend itself against a pushy tank mate. In the wild these loaches live in loose groups, and in a tank a lone kuhli is a stressed kuhli: keep five or six together and you will see them out in the open, woven through the plants in a relaxed tangle. Every tank-mate decision for this fish starts from those three facts: shy, soft-skinned, and social with its own kind.
Tank mates that suit a kuhli loach
The safest company lives above the kuhli's level and minds its own business. Small rasboras such as chilis and harlequins cruise the middle water and ignore the bottom entirely. Ember tetras and other tiny tetras do exactly the same. A honey gourami drifts slowly near the surface and is far too mild-mannered to bother a loach. If you want a second bottom fish, a small group of pygmy or panda corydoras is the usual success: they share the floor without competing hard, though you should feed deliberately so both groups get their share. Cherry shrimp are generally left alone by kuhlis, whose nocturnal foraging rarely disturbs them, and a mystery snail makes a slow, harmless floor companion too.
Fish and invertebrates to leave out
Anything that nips or investigates other fish with its mouth will eventually find a kuhli. Tiger barbs and serpae tetras are notorious for testing slow tank mates. Most cichlids, even the smaller species, claim bottom territory and will drive a loach out of its hiding place night after night until it stops eating. Boisterous loaches such as yoyo loaches can overwhelm a kuhli group at feeding time simply by being bigger and bolder. Crayfish hunt at night, exactly when kuhlis are active, and a scaleless fish makes an easy catch. Any fish large enough to swallow a slender loach, or a big-mouthed catfish that hunts the bottom after dark, rules itself out on size alone.
Arranging the tank so the mix works
Good tank mates still fail in a bare tank. Give the kuhlis a soft sand area for sifting, several caves or dense plant thickets, and smooth wood or leaf litter to slide underneath. Dim one end of the tank or float plants at the surface so there is always a shaded retreat available. Feed the daytime fish first, then drop a small sinking food close to the loaches' cover after lights-out, so the group eats without competition. When you add new fish, add them during the day while the kuhlis are hidden away, and watch the first few evenings with the room lights low to confirm nobody is being chased out of cover or going unfed.
Keeping the community settled
A kuhli tank changes slowly, and it is easy to forget which fish arrived together or which new addition sent the loaches into hiding for a week. For a pairing that still puzzles, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.