The most vulnerable fish in the house
Threadfin rainbowfish are good with other pets at home only when every barrier around them is doing its job. A threadfin is under two inches long, slow to feed, and decorated with fin rays that tear like tissue. To a cat it is an irresistible fluttering target; to many ordinary community fish it is either a snack, a toy to nip, or a competitor too slow to bother with. None of that is the threadfin's fault, and the species lives peacefully in a multi-pet home when the keeper accepts that this fish, more than most, survives on the quality of its protection.
Cats, dogs, and a lid that must not fail
A cat is a direct danger to a threadfin rainbowfish in a way it is not to larger, faster fish. Threadfins hover near the surface and plants rather than bolting, so a paw through an open feeding hatch or a gap around cables can actually catch one. Fit a complete, close lid, close it after every feed without exception, and give the cat no shelf from which to reach down into open water during maintenance. Dogs threaten the tank physically rather than the fish: a heavy dog barging the stand or leaping at movement can stress the school for days, so place the aquarium on solid furniture away from the dog's play route. Birds and small caged pets in the same room are no concern to threadfins at all.
Tankmates that will not nip, chase, or starve them
Inside the aquarium, choose threadfin companions by temperament before colour. Small, slow, peaceful species that feed gently, such as tiny rasboras, small peaceful tetras, pygmy corydoras, otocinclus, and ornamental shrimp, generally leave threadfins and their fins alone. Barbs, many larger tetras, bettas, and gouramis are risky in different ways: nipping at trailing fins, flaring at rival males, or sweeping up every particle before a threadfin has inspected it. Even other rainbowfish can be too fast and boisterous at feeding time. A species-only threadfin tank, or one shared with bottom dwellers that ignore them, is the combination most keepers find reliably peaceful.
Adding animals without breaking a fragile peace
Make changes around a threadfin rainbowfish slowly, because this species reads every disturbance as danger. A new kitten exploring the room will keep the school in the plants for days; keep the lid routine strict and feeding times fixed until the fish decide the new shape is scenery. New tankmates should be quarantined first, added under dim light in the evening, and then watched specifically at feeding time for a week, checking that each threadfin still rises and eats. Extra planting added at the same moment gives retreating fish cover and breaks the line of sight of any tankmate inclined to chase.
Signs the mix is hurting your threadfins
Shortening fins are the clearest early warning in a threadfin tank. If a male's trailing rays look trimmed, frayed, or white-edged, something is nipping or the water is failing him, and the cause needs finding that week, not next. A school that stops displaying, hovers only in one corner, or loses weight while tankmates fatten is being outcompeted or frightened. A cat discovered with a wet paw means the lid arrangement has already failed once and must be rebuilt, not merely closed more carefully. If several threadfins sicken after a new arrival, test the water and seek advice from an experienced aquatic shop promptly.
Peaceful, provided the protection is real
Threadfin rainbowfish fit a home with other pets when the aquarium is fully lidded, steadily placed, and stocked only with companions as gentle as they are. Treat their fragility as a design brief and the other animals in the house become background rather than threat. If the group is ever rehomed, KinsLe Transferrable Profiles sends the next keeper the list of tankmates the threadfins lived beside safely and the small foods they accepted, so a hard-won peaceful mix is not accidentally replaced with nippers and fast feeders.