The fish is never the problem in the household
A least killifish is peaceful in the extreme: it has no aggression to offer any other animal, no fin-nipping, no territorial chasing, nothing. It hovers in plants, picks at microscopic food, and minds its own business entirely. So when keepers ask whether least killifish are good with other pets, the honest answer reverses the question — the killifish will get along with everything, and the keeper's job is making sure everything else gets along with a fish small enough to be swallowed by most of the aquarium trade.
Inside the tank: tiny, calm companions only
Least killifish belong with tankmates matched to their scale and temperament: other very small peaceful fish, dwarf shrimp, and snails make suitable company, and a species-only tank shows their shy behavior best. Anything with a mouth the size of the killifish's body — which includes many fish sold as community fish — will treat them as food, and faster, pushier species will outcompete them at feeding time even if they never touch them. Dense planting is the peacemaker: it gives the killifish cover to retreat into, fry a place to survive, and every tankmate visual barriers that lower tension across the whole aquarium.
Cats: the one real household hazard
A cat is the other pet most likely to matter. Most cats simply watch a tank like television, which harms nobody, but an open-topped aquarium invites a dipping paw, and a determined cat on top of a light fixture or a flimsy lid can end badly for the fish and the furniture alike. A fitted lid or cover glass, a stand a cat cannot rock, and keeping the tank out of the cat's favorite leaping routes solve the problem almost entirely. If your cat can reach the tank, assume it eventually will, and set the tank up for that day.
Dogs and other animals in the room
Dogs are essentially irrelevant to least killifish beyond the risk of a big, boisterous dog colliding with the stand — place the tank where play does not happen and the issue disappears. Birds, reptiles, and small mammals in their own enclosures share a room with the tank without any interaction at all. The one cross-pet caution worth naming is equipment: never move a net, bucket, or plant between a reptile or amphibian enclosure and the fish tank without cleaning it, since residues and organisms travel on shared gear. Separate tools for the fish tank cost little and prevent strange problems.
Blending them into an existing multi-pet home
For a household that already keeps animals, least killifish are among the easiest additions possible: they need a small planted tank, a lid, a quiet corner, and tankmates chosen for gentleness rather than glamour. Introduce them after the tank is cycled and planted, feed them where the bolder fish will not monopolize the food, and watch during the first week for hiding that never relaxes — a settled least killifish hovers visibly among the plants at feeding time, while a bullied one simply vanishes into cover all day.
Keeping their place in the household records
In a busy multi-pet home, the smallest animals are the easiest to under-document, yet their feeding amounts, tankmates, and fry notes are exactly what another keeper would need. A written profile keeps their care from depending on one family member's memory. If your least killifish ever move to a new home, KinsLe's Transferrable Profiles send that profile — routine, tankmate history, and all — along with them, so their next keeper inherits the full picture.