How often should you actually replace your pet's water filter?.

Every pet water fountain ships with an instruction sheet, and every instruction sheet says roughly the same thing: replace the filter every two to four weeks. This is technically correct in the way that "a car should be serviced every year" is correct — it's a conservative default designed to protect the manufacturer from complaints, not to tell you when your specific filter is actually done.

The real answer is more useful: it depends on three things, and you can learn to read them yourself in about a minute.

Here is what those three things are, how to check them, and why the two-week rule is almost always wrong — in both directions.