An open aquarium is a hunting invitation a cat is built to accept: movement below a surface, small shapes darting, and water that ripples when patted. Owners often try scolding, spray bottles, or moving the cat down dozens of times a day, and lose, because the reward is live prey behaviour and the puddle resets every time they leave the room. The honest answer is that this problem is solved by hardware first and training second.
The Core Fix: A Lid That Actually Fits
Fit a secure cover made for your tank, whether that is a glass canopy, a mesh top, or a solid hood, and check it the way a cat will. Cats test edges with a paw and weight with their whole body. The lid should sit flush, resist a firm push from above and a hook from below, and cover the whole surface with no tempting open strip at the back.
Then block the gaps most owners miss: the cut-outs where filter pipes, heater cables, and airline tubing pass through. Foam, spare mesh, or a cut-to-fit cover piece closes these without strangling equipment. A determined cat fishing through a cable gap gets a paw, then stress for the fish, and sometimes a mouthful of a small fish or a mouth and gut full of tank water and gravel. If your tank came with an open-top rimless design, a made-to-measure mesh cover preserves the look while ending the hunting completely.
Keep the Cat Off the Top, Too
A lid rated for splashes may not be rated for a five-kilo cat napping on its warm surface above the lights. Teach the tank top as a no-go zone by making it unrewarding and making the alternative better: place a genuinely appealing legal perch, such as a window hammock or a cat tree, right beside the aquarium, so the cat can watch fish theatre for hours from a seat that is theirs. Reward the cat for choosing the perch, and lift them down calmly and immediately, every single time, when they choose the lid. Consistency here works because the alternative is good, not because the scolding is scary.
Never leave a cat alone in the room with a tank whose lid is off during maintenance. Water-change time is peak fishing opportunity, with the cover removed, buckets of tank water on the floor, and your attention on a siphon. Shut the cat out for the duration, every time, as a fixed habit.
Why the Fish Need You to Win This
Even unsuccessful fishing harms the tank. A paw in the water stresses fish into hiding, refusing food, and weakened immunity, and repeated tapping and lunging at the glass keeps shy species in permanent alarm. Cats can also knock heaters, lights, and tubing askew from above, and a cat who does get in can empty a surprising amount of water over the rim in a panic to get out. If your fish have gone off food or a fish has unexplained injuries since the cat discovered the tank, treat it as urgent: lid on, cat out, and check water quality, because stress and a disturbed tank often arrive together.
The lid rule fails only when a person leaves it off, and KinsLe Family Sharing puts the cover rule, the maintenance shut-out, and who may open the tank to feed in the shared notes for the household, so a helpful guest or child does not undo the protection while you are out.