Why cats fixate on open water
An open-top tank presses every button a cat has: moving shapes, rippling light, the occasional flicker of a fin at the surface, and water itself, which many cats find more interesting to dabble in than anything in their own bowl. A betta resting near the surface or shrimp grazing on floating plants are not just visible from above — they are at paw height for a cat sitting on the rim or the furniture beside it. Once a paw has connected once, even with nothing but water, the game is learned and the cat will return daily. Expecting the cat to lose interest on its own is expecting the tank to stop being fascinating, and it will not.
Cover the top without sealing the tank
The lasting answer is a physical barrier. A fitted glass or acrylic lid sized to the tank is the neatest option for a betta, whose tank needs the air above the water warm and humid anyway; leave the small feeding hatch shut rather than propped. For shrimp tanks, fine stainless or plastic mesh fixed over the top keeps paws and leaping shrimp on their own sides of the line while light and air pass freely. Weigh or clip any cover down — a determined cat sitting on a loose lid can push it in, turning a safety device into the accident itself. Leave no gap at the back where filter pipes pass through; cats work paws into spaces that look impossibly small.
Give the hunting instinct a better target
A cover stops the fishing, but the energy behind it still needs somewhere to go, especially in a young or indoor cat. Move the play to the floor away from the tank: a wand toy session at the times the cat usually stalks the aquarium burns off the same hunting sequence in the right place. A perch or shelf where the cat may legitimately sit and watch the tank — far enough back that a paw cannot reach the glass top — often satisfies the audience instinct without the participation. Rotate toys so the alternatives stay more interesting than the forbidden one.
Protecting shrimp and bettas in the meantime
While a cover is being made or delivered, treat the tank as unsupervised at all times. Close the door of the tank room when you are out, and move any chair, shelf or stack of books that gives the cat a launchpad level with the rim. Floating plants thick enough to hide under give shrimp a refuge from a dabbling paw, and a betta should have a cave or dense plant at the back, away from the cat's favourite fishing corner. Even a single swipe matters: claws carry bacteria, and a scratched fish or a shrimp carried out of the water can be lost from what looked like a harmless pat.
One rule for the whole household
Covers fail when one person leaves the hatch open after feeding or lifts the mesh just for a minute to do maintenance and forgets it. Agree the rule plainly — the top is closed unless hands are actively in the tank — and make whoever feeds or cleans the one who checks the cover is back before walking away. The rule works when it is written down where everyone can see it rather than carried in one person's head. Profile access for relatives you select is available using KinsLe Family Sharing.