Ctenocephalus Gastromyzon

Gastromyzon ctenocephalus is named for the comb-like serrations on its head profile, a feature visible in close-up photographs and lending it a slightly prehistoric appearance. It inhabits the clear, fast-flowing hill streams of Sarawak and Sabah in Malaysian Borneo, where it grazes algae and biofilm from smooth boulders submerged in powerful current. Like all Gastromyzon, it is adapted to water that is cool relative to typical tropical temperatures and perpetually saturated with dissolved oxygen.

In the aquarium, this species settles well in a hillstream biotope with adequate flow and smooth substrate. It is moderately tolerant of similarly sized hillstream species but males may display brief chasing behaviour when territories overlap on prime grazing surfaces. Females are somewhat plumper when in breeding condition.

Gastromyzon species from Borneo generally prefer slightly warmer conditions than their Vietnamese counterparts — 22–26 °C is appropriate — and they do not require quite as extreme a current as Sewellia. This makes G. ctenocephalus marginally more accessible for keepers who struggle to maintain sub-20 °C temperatures.

Water: 22–26°C, pH 6.0–7.0, GH 3–12, KH 1–6; high flow and strong oxygenation essential; avoid any organic waste accumulation. Tank: 80 L minimum; smooth rounded pebbles and boulders; avoid sharp substrate; keep silt to a minimum with good mechanical filtration. Feeding: Biofilm, microalgae, and diatoms; supplement with algae wafers, spirulina tabs, and occasional blanched vegetables. Breeding: Rarely bred in captivity; eggs deposited under smooth stones; increased flow and slight temperature drop may condition adults. Compatibility: Peaceful with non-hillstream community fish; mild territorial interactions with conspecific males are normal.