A water filter wrench is a plastic or metal strap or cup that grips the sump of a filter housing so you can unscrew it. They are not universal: the cup type matches one housing diameter and profile, so the correct question is which housing you own rather than which wrench is best.
Cup, strap and band types
The cup wrench slides over the bottom of the sump and matches its diameter and rib pattern, giving the most even grip and the least chance of cracking the housing. A strap or band wrench tightens around any diameter and is the fallback when you cannot find a matching cup, though it can crush a thin sump if you overdo it. Most manufacturers supply a cup wrench in the box, which is why the first step is looking in the drawer where the paperwork went.
Match it to the housing you have
Slim line housings around two and a half inches across and heavy duty housings around four and a half inches take different wrenches, and even within a size the rib spacing varies by brand. Take the old wrench or the housing model number when you buy. If the housing has no model marking, measure the outside diameter of the sump and count the ribs, then compare against the manufacturer's spare parts listing rather than guessing from a photograph.
Why the housing is stuck in the first place
Housings seize for two reasons: the sump was overtightened at the last change, and the o-ring was left dry. Scale and iron deposits then lock the threads. Penn State Extension gives recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese as secondary aesthetic standards, and water carrying either leaves the orange or black deposit you find crusted on the threads. Cleaning the groove and lubricating the o-ring with silicone grease approved for potable use prevents the next seizure.
Working safely on a stuck housing
Close the inlet valve, then open the pressure relief button on the cap before you apply any force, because a housing under line pressure can let go suddenly and soak the room. Support the housing bracket with one hand rather than letting the wrench twist it off the wall. If a cup wrench slips, warm water on the sump and steady pressure beat a longer bar every time; brute force on a plastic sump splits it and turns a cartridge change into a replacement.
Questions people ask about water filter wrenches
Is there a universal water filter wrench?
Strap and band wrenches come close because they adjust to any diameter, but a matching cup wrench grips more evenly and is far less likely to crack the housing.
How do I open a filter housing without a wrench?
Relieve the pressure first, then try a rubber grip pad or a strap wrench. Avoid pipe wrenches and screwdrivers, which split plastic sumps.
How tight should the housing be after a cartridge change?
Hand tight with a lubricated o-ring is enough. Extra force on the wrench is what makes the next change difficult and deforms the sealing surface.