These are replacement body o-rings for Arkal Disc Filters.
If the body of the filter leaks where the two halves connect together check the body o-ring for damage. The o-ring should be removed, cleaned and inspected periodically. If dry, it should be lubed with o-ring lubricant. If damaged, it should be replaced. O-ring lubricant will prolong the lifespan of o-rings and help create a water tight seal.