Rummynose Tetra (Hemigrammus bleheri)

That bright red nose isn't a fixed marking, it visibly fades when the fish is stressed or the water quality slips, which is exactly why experienced keepers treat a rummynose school as a living water-test kit. Hemigrammus bleheri and its close relatives are also some of the tightest, most precisely synchronized shoalers in the hobby, moving as a single block rather than a loose cluster, a behavior studied as a model for collective motion in fish schools generally. Worth keeping a real group just to watch that synchronized movement in person.

Care level: Moderate · Temperament: Peaceful, shoaling · Max size: ~2" · Min tank: 20 gal · Diet: Omnivore, flake/micro-pellet