When reinstalling filters first hold the cartridge underwater until the air stops bubbling (at this point most air has been removed from the cartridge). Then screw the cartridge back into place in a clockwise direction.
NOTE: DO NOT screw the cartridge back into place excessively tight. Only screw the cartridge until it no longer spins freely. The suction from the pumps will pull the cartridges in further once operated. If the cartridge is screwed in place excessively tight before pump operation, the cartridge may be difficult to remove or could even cause the filter thread connection to twist or break the next time the cartridge is removed and breakage is not covered by warranty.