Gastromyzon ctenocephalus

Easy with the right water, quick to die with the wrong water. It's a Bornean torrent hillstream loach with three non-negotiables: cool temperatures (19–23 °C, must stay below 26 °C in summer), high oxygen, and strong current. Meet all three and it's a tireless algae grazer — let summer heat pass 26 °C and it dies off one by one with no warning. That's the number-one failure scenario. It's a close cousin of the Hong Kong hillstream loach (Pseudogastromyzon) with near-identical care.

Same family (Balitoridae), similar form and habits, different origins: Gastromyzon is distributed across Borneo (Malaysia, Indonesia), while Pseudogastromyzon ranges through southern China and northern Vietnam. Gastromyzon is flatter with a broader sucker mouth and dark-brown worm-line patterning; Pseudogastromyzon is slightly slimmer and gray-yellow. Tank requirements are identical: cool water, high oxygen, strong flow.

Yes, and it's often more diligent than small plecos on glass. It grazes with a sucker mouth, spending all day pressed to glass, stone, and wood eating algae and biofilm. Two caveats: it eats algae, not fish waste, and once the glass is clean you must supplement algae wafers; at 3–4.5 cm it's efficient only in small to mid-size tanks. It's largely ineffective against BBA and staghorn.

Simulate a mountain stream: smooth cobblestones stacked into crevices and caves, strong current running through the rock pile, soft diffused light. Use fine pebble or creek sand — sharp edges abrade the abdominal sucker. The key is attachment surfaces: a bare glass tank actually unsettles it; rock and wood surfaces everywhere are its comfort zone. Keep lighting subdued — it dislikes bright light.

45 L is the floor; 60–80 L in practice. It's small but oxygen-hungry and current-dependent; larger water holds temperature steady (higher summer safety). A small 45 L spikes dangerously in summer heat — bigger water plus a fan or chiller is the long-term answer.

Algae and biofilm are staples, but tank algae rarely sustain it long-term. Supplement: quality sinking algae wafers every other day, blanched spinach or pumpkin slices, and occasional frozen brine shrimp for protein. Hunger test: a hollow belly and glass-edge grazing all day means it's underfed.

Only fish sharing its water demands: white cloud mountain minnows, medaka, small barbs, juvenile zacco. The rule: tankmates must also love cool water and high oxygen. Standard tropical fish are out — guppies, tetras, bettas all fail on temperature. A gentle-looking neon tetra is a slow death sentence at 26 °C.

Probability order: underfeeding (insufficient algae, no wafer supplement); chronically warm water (above 26 °C disrupts metabolism and wastes the fish); weak current (chronic low oxygen); intestinal parasites (common in imported, wild-caught stock). Fix: feed more first, then check temperature and flow, and quarantine-deworm if those fail.

Yes, and it's good at it. Hillstream instinct runs downstream and leaps waterfalls — any open water surface is a runway. A lid is mandatory; loose top-mesh schemes aren't enough, it finds gaps and ricochets out. Jumping drive peaks in the first two weeks after introduction.

It's from Bornean mountain torrents: crystal-clear, oxygen-saturated, cool (20–24 °C year-round), fast-flowing, with algae- and biofilm-covered rock surfaces. That environment dictates every aquarium requirement — it's a 'flow-environment indicator species', the first casualty when current and oxygen fall short. Understand the wild habitat and the strict temperature and oxygen demands make complete sense.