Glass Catfish (Kryptopterus sp.)

Glass catfish take transparency about as far as a vertebrate can: skin, muscle, even the ribcage are visible edge to edge, with only the swim bladder and spine reliably opaque. That swim bladder is oddly shaped too, stretched into a long thread reaching well back into the body rather than the compact oval most fish carry, likely fine-tuning buoyancy control for a fish that spends its life hovering in loose midwater groups.

Care level: Moderate · Temperament: Peaceful, shoaling · Max size: ~4" · Min tank: 30 gal · Diet: Carnivore, small live/frozen foods