If you own an AQ-500 "Big White" Filter your O-Ring will be squared.
If you own an AQ-400 series Countertop Unit your O-ring will be rounded.
If you own an AQ-100 series Under the Sink Unit your O-ring will be rounded.
NOTE: The Countertop O-ring is slightly larger than the Under-the-Sink O-ring. Please use your model number as refrence for selecting the correct size O-ring.
Proper maintenance of O-rings will ensure that no leaks occur. O-rings are what create the watertight seal in the housings and are essential for preventing leaks. When tightening the lid onto the housing, it only needs to be hand tight as the O-ring will provide the watertight seal, not the threads of the lid. In order for the O-ring to maintain a proper seal it is extremely important that it remain lubricated with a silicone gel. All O-rings, both new ones that come with your unit, and replacement O-rings will come pre-lubricated. NOTE: PLEASE DO NOT WIPE OFF THE SILICONE LUBRICANT FOR THE O-RING, IT IS ESSENTIAL TO MAINTAIN WATERTIGHT SEAL. Also, please do not stretch the O-rings. They may look like similar to a rubber band but they are NOT meant to be stretched. If an O-ring is stretched it can't be "unstretched," therefore you will need a new one.
When screwing in the housing after replacing a cartridge, always have the opening facing up. For Countertop units this means flipping the base over to make sure you are screwing it in with the opening facing up. Please do not screw in the housing with the opening facing down as the O-ring can fall out of it's groove. If the O-ring falls out during tightening, it can get damaged, which means you will need to get a new one.