Fish and small caged pets are fascinating to a cat in a way few household objects are: they move, they make small sounds and they sit behind glass or bars at exactly a cat's eye level. That fascination does not have to be a problem, but it is managed by engineering first and training second. The enclosure must be safe on the worst day, when she leaps at it unsupervised mid-chase, not merely on the calm days when she watches politely. Once the hardware cannot fail, you can teach her that the residents are background scenery, boring by design, while the rewards in the room come from you.
Proof the Enclosures First
Before she enters the room, fit a latching, weight-bearing aquarium lid that stays shut and stays up when a full-grown cat lands on it, because a lid that flexes, slides or dips into the water is an accident scheduled rather than prevented. Move filters, heaters and lights so their cables run out of paw reach, with no dangling leads to chew or pull equipment down. For caged small pets, choose narrow-bar cages she cannot reach through, and set them on stands that cannot rock, wide-based and steady enough that a cat brushing past or rearing up cannot tip or rattle the cage into a panic. Raise cages off the floor where she patrols, keep cage doors latched with the same seriousness as the tank lid, and check every fixing with a firm push and pull of your own. Only when you would trust the setup overnight does the second stage begin.
Teach Calm Watching at a Distance
Let her into the room with the enclosures closed and stay with her, starting at a distance where she notices the fish or the small pets but can still turn away, eat a treat and look back at you. Reward calm watching the moment it happens, with food delivered beside you rather than at the glass, so turning away from the tank becomes the behaviour that pays. Give her a perch of her own nearby, a tree or shelf at a similar height with a good view of the room, because a cat with a legal seat near the action is far less likely to claim the tank lid as hers. Keep early sessions short and leave while she is still relaxed. Block the fixating behaviours early and kindly: redirect pawing at glass, crouching with a lashing tail or chattering fixation with play away from the enclosure, and end the visit if she cannot settle, since rehearsing a stalk at the glass teaches the opposite lesson.
Supervision Until Calm Is Proven
Access stays supervised for a long probation. She gets no unsupervised access until weeks of calm behaviour have accumulated, meaning repeated visits where she glances at the residents, chooses her perch or a toy instead and can be called away without drama. Count the pattern, not a single good afternoon, because one startled hamster dash or one fish jumping at feeding time can reset her interest to full hunting mode. Keep doors closed between her and the enclosures whenever nobody is in the room, and brief every household member and visitor on the same rule so a guest never props a door open for her. Some households, sensibly, never graduate beyond this stage and keep the room permanently closed, which harms nobody. Wherever your household lands, the written rule survives memory because it is recorded through Family Sharing in KinsLe, alongside which stage of supervision she has reached.