Gastromyzon ctenocephalus

Gastromyzon ctenocephalus has a flattened body adapted to life on rocky substrates in fast-flowing water. The base colour is brown, with small cream-yellow spots covering the body. The fins have a striped pattern with a bluish tint. It differs from related species in having blue markings on the dorsal fin and continuous horizontal blue stripes on the caudal fin. Adults reach 3.5–5 cm in length. Sexual dimorphism is weak: females are typically rounder and bulkier than males.

This species is endemic to the western part of Borneo (Malaysia and Indonesia). It inhabits shallow mountain streams with fast flow, clear water rich in oxygen. In nature it occurs on rocky, gravelly or rocky substrates where a rich biofilm of algae and microorganisms forms on the stone surfaces.

A peaceful but territorial fish. In the aquarium it lives on the bottom, preferring to stay on stones or glass using its fins as suckers. Skirmishes between individuals over feeding sites may occur, but these are usually ritualistic and cause no injury. A group of 4–5 individuals is recommended to observe natural social behaviour.

A tank of at least 60 litres is needed with powerful filtration and aeration to provide strong water flow and high dissolved oxygen levels. Furnish the tank with smooth stones, pebbles and boulders to mimic a mountain stream bed. Lighting should be bright enough to stimulate algae growth on the stones. Water parameters: Temperature 20–24 °C, Water pH 6.0–8.0, Water dH 2–12.

In nature it feeds mainly on algal biofilms and microorganisms. In the aquarium the diet should include specialized plant-based pellets for herbivorous fish, sinking tablets for bottom dwellers, and blanched vegetables such as spinach or courgette. Frozen foods such as bloodworm or brine shrimp can be offered occasionally, but the diet should be plant-based to prevent digestive problems.

Breeding in captivity is extremely difficult and rarely occurs. Spawning is paired or in groups, with eggs laid on stones in areas of strong flow. Fry need abundant biofilm on stones to feed on in their first days. No proven methods for reliably stimulating breeding have yet been developed.

This fish is sensitive to water quality and dissolved oxygen levels. The main risks are nitrate accumulation and insufficient aeration, which quickly lead to a depressed state. Poor keeping conditions may cause bacterial infections and internal parasites. Prevention consists of maintaining clean water and regular water changes.

When buying, choose active individuals that hold themselves confidently on glass or stones. Avoid fish showing signs of emaciation, sunken bellies or torn fins. Since this species is often imported stressed, a quarantine period and conditions with high oxygen concentration in the tank are recommended for adaptation.