The clown killifish, a banded fish barely an inch long with a tail like a painted rocket, lives its whole life in the top layer of the water, hunting under floating leaves in the still shallows of forest streams. Its tank is therefore measured differently from most: surface area, overhead cover, and lid fit matter more than total gallons, and a small, low aquarium set up correctly beats a large, deep one set up casually.
Five to ten gallons, arranged shallow
A tank of about 5 gallons suits a pair, and 10 gallons suits a small group with room for fry to grow up alongside the adults, which they will do happily in a well-covered setup. Choose the longest, lowest shape available rather than a tall cube, because the fish use the surface the way other fish use the whole water column, and extra depth below them goes unused. Water level can sit a little below the rim, which also helps with the species' defining hazard.
That hazard is jumping. Clown killifish launch themselves through gaps that look impossibly small, around cables, filter cut-outs, and feeding hatches, so the lid must be complete and its gaps blocked with mesh, foam, or craft plastic. Treat every opening as a door until you have physically checked it, and re-check after any equipment change.
A roof of plants over still water
Floating plants are not decoration for this species but the habitat itself. A mat of floating roots and leaves gives the fish hunting shade, a place for the female to place eggs among fine roots and moss near the surface, and the security that brings them into view. Add fine-leaved plants and moss just under the surface, a dark substrate below to set off their banding, and keep flow genuinely gentle, since a current-swept surface exhausts a fish built for still pools.
Sponge filtration suits the scale of the tank and the fish, adding no dangerous intake gaps. Lighting can stay modest; the floating canopy will soften it further, and the fish show their best colour in that dappled shade rather than under a bare bright light.
Small tank, close attention
The trade for a tiny aquarium is vigilance, because a few gallons swing in temperature and waste faster than a large volume forgives. Feed small live and frozen foods at the surface in amounts the group finishes quickly, watch that every fish, including the smallest male, gets a share, and keep changes small and frequent rather than rare and large. In return the tank gives you behaviour large aquariums rarely show at this intimacy: courtship at the surface, eggs in the roots, and fry appearing in the canopy like punctuation.
In a group tank the surface divides into small, defended patches beneath the best root clusters, so spread the floating cover in several rafts rather than one mat and each male can hold a station without constant chasing across the top.
Because the lid is the whole safety system, make its check a shared habit rather than one person's memory. Listing the gap seals and the after-feeding lid check in KinsLe Family Sharing means whoever feeds the killifish, family or guest, closes the tank up the same careful way.