Short answer: every two to four weeks. Closer to two if you have more than one cat, a long-haired cat, or hard water. Closer to four if you have one short-haired cat and soft water.
That range is not a manufacturer trying to sell you filters. It is roughly how long the activated carbon inside stays active.
What a filter actually does
A decent fountain filter has three jobs, usually in three layers:
- A mesh screen catches fur, litter granules and food crumbs before they reach the pump. This is the layer that stops your pump dying.
- Activated carbon adsorbs chlorine and the compounds that give tap water its taste. This is the layer that decides whether your cat drinks.
- Ion-exchange resin softens hard water and slows the mineral scale that eventually seizes a pump.
The mesh keeps working until it clogs. The carbon does not. Once its surface area is saturated, it stops adsorbing, and from that point it is a wet sponge sitting in your cat's water.
Five signs yours is overdue
- Your cat has gone back to the tap. This is the clearest signal and usually the first. Cats notice a change in water taste long before you do.
- The flow has weakened. A clogged filter starves the pump.
- The pump is louder. Same cause. It is working harder for less water.
- You can see debris sitting on the filter surface, or a slick film on the basin.
- The water smells of anything at all. Clean fountain water should smell of nothing.
What happens if you do not change it
Nothing dramatic on day one. Over weeks, three things go wrong at once. The water starts tasting of whatever the carbon stopped catching, so your cat drinks less. Biofilm builds on the filter and basin, which is a slimy bacterial layer you can feel with a fingertip. And the pump grinds through mineral scale and hair until it fails, usually somewhere between six and eighteen months early.
The first of those is the one that matters. The entire point of a fountain is to raise your cat's water intake. A fountain your cat has quietly stopped using is worse than a bowl, because you think the problem is solved.
Do this at the same time
Changing the filter without cleaning the fountain is half a job. Every time you swap the filter:
- Empty and rinse the basin, and wipe it with a soft cloth rather than an abrasive pad.
- Pull the pump apart. Most disassemble into three or four pieces. There will be a surprising amount of hair inside the impeller housing.
- Rinse everything in hot water. Skip soap unless you are certain you can rinse every trace of it, because cats will refuse water that carries detergent.
- Refill and run it for a minute before you walk away, to be sure the pump has primed.
A cheap way to never think about it
Buy filters in packs rather than singly, and put a recurring reminder on your phone for the first Sunday of the month. That is genuinely the whole system. The people who keep fountains running for years are not more diligent than everyone else, they just took the decision out of their own hands.
This article is general guidance, not veterinary advice. If your cat has suddenly stopped drinking, is straining in the litter box, or is urinating more or less than usual, speak to a vet rather than changing a filter.