Honesty first: a cat is a predator in this house
Every cat carries hunting instincts, and in a home that already keeps fish, hamsters, gerbils, budgies or similar small pets, the question is not how to remove those instincts but how to choose a cat whose drive is manageable and build a setup that never tests it unfairly. Small animals moving quickly behind glass or bars are intensely interesting to most cats, and being stared at, pawed at and lunged toward is frightening for the small pet even when no physical contact happens. A household wanting everyone loose together in one room should not add a cat at all.
The cat to choose: settled, low-drive and proven
Your best candidate is an adult cat known to be placid, ideally one whose fosterer keeps small animals and can describe her actual behaviour around them: a glance and a wander away is what you want. Older, settled cats who prefer laps and windowsills to ambush games are generally safer than adolescents. Kittens are the riskiest choice, not because they are wicked but because their prey drive strengthens as they mature and their play is fast, physical and impossible to assess reliably at eight weeks old.
Be cautious with cats described as keen hunters, prolific mousers or highly reactive to birds through windows. That brilliance outdoors becomes a full-time problem indoors when the quarry lives in a cage across the room. Ask the rescue direct questions, and believe the answers.
Tanks and cages: barriers, not good intentions
Fish tanks need rigid, lockable or heavily weighted lids the cat cannot shift, because cats drink from, paw at and occasionally fall into open tanks, and a determined cat can dislodge a light cover. Position the tank so there is no launch pad beside it from which the cat can sit on the lid for hours, stressing the fish with her shadow and tapping.
Caged mammals and birds belong in a room the cat does not enter unsupervised, with a door that actually latches, as many cats learn lever handles. Cages themselves should be sturdy, with bar spacing no paw fits through, catches a paw cannot flip, and no spot in the home where the cage sits on a surface the cat can jump onto and topple it. Free-roaming exercise time for the small pet happens with the cat shut in another room, every time, without exceptions for a cat who seems to be asleep.
Giving the cat a better job to do
A cat whose hunting energy has nowhere to go will spend it on your smallest pets. Redirect it deliberately: two proper play sessions a day with wand toys she can chase, catch and finish, puzzle feeding that asks her to work for part of her dinner, and a window perch or shelving positioned away from the fish tank so watching the world competes with watching the gerbils. End play with a small meal to complete the hunt, catch, eat sequence that settles hunting cats most deeply.
Watch for escalation signs: the cat camping outside the small-pet room door, waiting to dart in, fixating despite redirection, or becoming agitated at the cage.
Who checks what, every single day
In mixed-pet homes, accidents are almost always routine failures: a lid left unlatched, a door left ajar by a guest. Make the checks explicit: lids secured after every tank interaction, the small-pet room door pulled to and tested, the cat located before any small pet comes out, and play sessions delivered so her hunting drive is spent. KinsLe Care Teams lets your household assign those lid, door and play checks to named people and mark each one complete with a timestamp, so the routine protecting your fish and caged pets never depends on anyone's memory.