Inside the tank: companions must be tiny and gentle
Start with the neighbours clown killifish actually live with. They are small, peaceful, surface-dwelling fish, and their mouths and temperament set narrow limits. Suitable companions are other tiny, calm species that stay out of the surface layer, along with snails and adult shrimp, in a tank sized for everyone. Even then, expect the killifish to hunt baby shrimp, because a newly hatched shrimplet is exactly the size of food they are built to take at the surface.
The reverse risk is bigger. Any fish large enough to fit a clown killifish in its mouth eventually will, and even medium community fish that merely chase or outcompete at feeding time will starve these shy surface feeders slowly. Boisterous tankmates are as incompatible as predatory ones, just quieter about it.
Cats: the lid is the whole answer
Most cats lose interest in a well-run fish tank once the novelty fades, but clown killifish raise the stakes because they jump and they live at the surface, exactly where a cat watches and pats. A tight, complete lid that a cat cannot shift or dip a paw through turns the tank into television; an open or flimsy top turns it into a fishing spot, and a cat sitting on a warm light fitting adds a real risk of pushing it in.
Give the cat a better perch nearby if the tank becomes a favourite seat, ideally one that still lets the cat watch the fish from a comfortable distance, and never rely on telling the cat no. With the lid secured, cats and clown killifish coexist uneventfully in many homes for the fish's whole life.
Dogs, birds, and small mammals: distance and stability
Dogs rarely threaten the fish directly, but a large, enthusiastic dog can rock a small tank stand, splash water over electrics, and keep shy fish pressed into the plants with constant close investigation. A solid, level stand the dog cannot shove, in a spot the dog passes rather than camps in, solves most of it. Free-flying birds and roaming small mammals matter mainly around open-top maintenance moments, so tank jobs happen with the room door closed and other pets elsewhere.
The pattern across all of these is the same: clown killifish are unbothered by other pets they can only see, and badly served by any arrangement where contact is possible. Design for separation once, and no daily vigilance is needed afterwards.
A quick household audit before you buy
Walk the route before the fish come home. Is the stand solid, the lid complete, the cat's favourite route across furniture kept off the tank, and the dog's play space somewhere else? Does everyone in the house, including visiting children, know the lid stays shut? If those answers are yes, your other pets are no obstacle at all to keeping clown killifish happily.
Writing those arrangements down where the whole household can see them helps them stick, and KinsLe Family Sharing shares your killifish's profile and care information with the family who help, so everyone works from the same details on lids, feeding, and keeping other pets back.