Knifefish

Freshwater knifefish, 26 listings across the three tiers, and the group splits cleanly in two. The Chitala — Clown, Gold Clown, Royal — are ambush predators that get genuinely large and will eat tankmates that fit. The ghost knives, Black Ghost and Brown Ghost, are nocturnal and navigate by a weak electric field, which is why they object to crowding and to being kept in the open.

If you run a normal six-foot tank, the African Knife (Xenomystus nigri) is the one that actually fits; a Clown Knife outgrows almost every home aquarium and ends up rehomed. The ghost knives are effectively scaleless and take standard medications badly — which is the real argument for buying these already quarantined rather than treating them at home.