Glass Catfish Shoal Behaviour Guide: Kryptopterus Group Care
Glass catfish are one of the few shoaling catfish that truly shoal — tight, ordered, and synchronised, hanging at an angle in midwater as if suspended on fishing line. This glass catfish shoal behaviour guide focuses on the group dynamics that most sources skip, drawing on Gensou Aquascaping’s long-running Kryptopterus vitreolus display in Everton Park. Keep fewer than eight and you will see a very different fish — skittish, pale, hiding — which is the most common complaint with the species.
Species Sold as Glass Catfish
Two species dominate the trade. Kryptopterus vitreolus (formerly mislabelled K. bicirrhis for decades) is the small, 6-7cm Thai species most shops carry. Kryptopterus minor is smaller still. The larger, more aggressive K. bicirrhis reaches 15cm and is rarely imported. Price difference is modest, but temperament is not — always check size at purchase.
Minimum Shoal Size
Eight is the absolute floor. Twelve or more is substantially better, and groups of twenty display the classic rolling formation that lazy shoals never achieve. In smaller groups, the fish form rigid social hierarchies where one individual dominates the shoal’s posture and the rest fade. Large shoals spread the social pressure and all fish display equally.
Tank Dimensions
Footprint matters more than volume. A 90cm long tank at 150 litres outperforms a 45cm cube of the same volume every time. Glass catfish cruise horizontally and need running room. Depth under 45cm keeps the shoal visible rather than tucked into background shadows.
Lighting and Colour Display
Their transparent bodies transmit and refract light beautifully under moderate lighting. Too bright and they retreat; too dim and the iridescent flashes vanish. Aim for 0.3-0.5 watts per litre of LED, with floating plants breaking the top third. A dark substrate and dark background turn the shoal into a moving gallery.
Flow and Positioning
In nature they live in slow but steady riverine flow. In the tank, a spray bar or directional filter outlet producing 1-3cm per second encourages shoal orientation. Too much flow scatters them; dead water makes them drift listlessly. Our glass catfish care guide covers baseline husbandry in more detail.
Water Parameters
pH 6.5-7.5, GH 4-12, temperature 22-26°C. They handle Singapore tap water well after dechlorination. Nitrate tolerance is moderate — keep it under 20 ppm. Sudden parameter swings cause the entire shoal to become translucent white within minutes, a stress response that signals impending disease if not addressed.
Feeding the Shoal
Kryptopterus are mid-water suspension feeders. Sinking pellets are mostly ignored. Offer small frozen foods — bloodworm, artemia, mysis — suspended in the current so the shoal can pick them off in passing. Live daphnia is the single best foundation food. Flake food works only if crumbled fine and poured into moving water.
Tank Mate Selection
Pair with peaceful, mid-to-upper-water species that occupy different layers — harlequin rasboras, pencilfish, small corydoras, otocinclus. Avoid anything fast or aggressive at feeding, including most barbs and danios, which outcompete the catfish for food and cause shoal dispersal.
Health and the Stress Response
Glass catfish turn opaque when seriously stressed. New arrivals often look milky for 48-72 hours — this is normal acclimation. Persistent opacity beyond a week signals internal parasites, which require metronidazole-based treatment at low dosage. They are sensitive to copper and salt, so standard fish-store dewormers are often too aggressive.
Quarantine and Import Stress
Most imports arrive heavily stressed after long bag times. A proper 2-3 week quarantine with dim lighting, no tankmates, and live food builds their reserves before they enter the display. Losing half a shipment in week one is unfortunately common if this step is skipped.
Long-Term Shoal Maintenance
Individual fish live 6-8 years, which is long for small catfish. Shoal cohesion weakens as older members die and the group drops below the critical mass. Top up proactively — bring in three or four fresh additions every 18-24 months to keep the shoal’s social structure strong.
Related Reading
emilynakatani
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