DIAGNOSIS The large white O-ring on the filter head is dirty, damaged, misaligned, or the housing isn't tightened enough.
FIX Shut off water and depressurize. Unscrew the housing. Inspect the O-ring seated in the filter head groove. Clean it and the mating surface on the head. Re-seat the O-ring firmly in its groove. Hand-tighten the housing, then use the wrench for an additional quarter-turn—no more.
IF THAT DIDN'T WORK The O-ring may have a flat spot or cut. Replace it with Aquasana O-ring kit 304010-60. Lubricate the new O-ring with a tiny bit of food-grade silicone grease (not petroleum jelly).
HEADS UP Overtightening is as bad as under-tightening. It can distort the O-ring and cause leaks. Hand-tight plus a quarter-turn with the wrench is the spec.