I have one I find if the pump is switched off as it would / should be ! I open a tap because if you don't it can cause a vacuum and makes it even more difficult to separate the 2 halfs .
As the filter cartridges are now really expensive, I have been to a Boat dealer and I am considering changing the system as the one I looked at there the cartridges are £28 instead of £70 to £80 ,I did email General Ecology the owners of Nature Pure about the cost of cartridges and got an email back saying the email had been passed to the Vice President of Sales !!!!??? I replied to that email saying I look forward to hearing from you about the said email ,heard nothing since never been a great lover of American ran businesses.
Tony A