Densely scaled Yunnan loach

Where they come from

Yunnanilus polylepis comes from Yunnan, China - cool, clear streams and small rivers up in the hills. Think rocky bottoms, lots of oxygen, and water that stays on the cooler side most of the year. That background explains basically every quirk they have in aquariums.

Setting up their tank

This is an advanced fish mostly because they are picky about water quality and oxygen. You can keep them in a smaller footprint tank, but they look and behave way better in a long tank where flow matters more than depth.

• Tank size: I would start at 20 gallons long for a group, bigger is easier to keep stable
• Temperature: cool to mid range (around mid 60s to low 70s F). I avoid keeping them in warm rooms without a plan
• Filtration: strong, reliable biofiltration plus surface agitation. A canister or a good HOB and a sponge prefilter works well
• Flow: moderate. Not a blender, but they do better with a directional current across the bottom
• Substrate: smooth sand or very fine rounded gravel. They poke around and you do not want sharp stuff
• Hardscape: rounded river stones, cobbles, small caves, and leaf litter in calmer corners
• Plants: tough plants in sheltered spots (Anubias, Java fern, moss). In higher flow, they tend to get battered

They really reward a mature tank. If you can stand it, let the tank run a couple months, grow some biofilm, and get the micro-life going. New, sterile setups tend to make them hide and pick at food less confidently.

What to feed them

Mine have always been enthusiastic micro-predators and pickers. They will graze biofilm, but they do not live on algae. The trick is small foods, offered in a way that reaches the bottom without blowing all over the tank.

• Staples: live or frozen daphnia, cyclops, baby brine shrimp, chopped bloodworms (sparingly), blackworms if you can get them clean
• Prepared: sinking micro-pellets, small soft granules, crushed high quality flakes (so it sinks), gel foods pressed into crevices
• Natural grazing: let rocks and wood grow a little biofilm; they will work it all day

Watch their bellies. A well-fed Yunnanilus looks nicely filled out behind the head, not pinched. If one fish stays skinny while others look fine, that is usually either internal parasites or it is getting outcompeted.

How they behave and who they get along with

They are not a "clown loach" personality fish. More understated. In a comfortable tank they spend a lot of time cruising the bottom and perching on stones, with short little bursts of speed. They also do that classic loach thing of wedging into cracks you swear are too small.

Keep them in a group. Singles get shy and you will barely see them. In a group, you get more natural behavior and they are bolder at feeding time.

• Good tankmates: other cool-water, peaceful stream fish (small danios, white cloud mountain minnows), small hillstream loaches in a big enough tank, peaceful small barbs that like cooler water
• Avoid: big boisterous fish, fin nippers, warm-water species that want 78-80F, and anything that will camp the bottom and bully them
• Shrimp/snails: usually fine with adult shrimp, but expect babies to disappear. Snails are generally ignored

Breeding tips

Breeding is possible, but it is not a "drop in a spawning mop and boom" situation for most people. The best luck I have had (and seen others have) comes from conditioning heavily, keeping them cool, then giving them a seasonal nudge with cooler water changes.

• Start with a group (6+). Sexing is not super obvious until they are mature
• Condition with lots of small live/frozen foods for a few weeks
• Use a tank with fine-leaved plants or moss, plus rock crevices. Eggs tend to get scattered
• Do a series of cooler water changes to mimic rain and a temperature dip
• If you see spawning behavior, consider pulling adults after - they will snack on eggs and fry

Fry, if you get them, are tiny and need infusoria-style foods at first (or very small commercial fry powders), then baby brine shrimp once they can take it. A mature tank with microfauna helps a lot.

Common problems to watch for

Most losses with this species come from "looks fine, then crashes" issues: heat, low oxygen, and instability. If you keep them cool, clean, and well-fed, they are fairly sturdy.

• Heat stress: they get lethargic, breathe fast, hide more, and stop competing for food
• Low oxygen: hanging near surface, rapid gill movement, sitting directly in the strongest flow
• Skin and barbel damage: from sharp gravel or dirty substrate; shows up as redness or worn mouths
• Internal parasites: chronic thinness, stringy poop, fading appetite (quarantine new fish)
• Ich and other spots: can happen after shipping stress, especially if they came in warm and were moved to cooler water too fast

My routine that keeps them out of trouble: weekly water changes, rinse prefilters before they clog, keep the tank from creeping hot in summer, and feed small amounts often. If you do those boring basics, these little loaches turn into a really rewarding, always-moving stream fish.