DIAGNOSIS The filter housing has a hairline crack, usually from overtightening during a filter change or a manufacturing defect in the polypropylene shell.
FIX Replace the housing sump. Order Aquasana part number 304010-59 (the 3/4" housing). Shut off water supply and relieve pressure. Unscrew the old housing with the filter wrench, install the new one, hand-tighten plus a quarter-turn with the wrench. Don't crank it.
IF THAT DIDN'T WORK If the new housing also leaks, the O-ring on the filter head (the part the housing screws onto) is damaged or misaligned. Inspect it, clean the seat, and ensure the O-ring is present and seated correctly.
HEADS UP The housing is under constant pressure. A crack will worsen. Don't try to seal it with epoxy or tape—it's a pressurized food-contact part.