Update: The community tank is now at around 6dGH. dKH holding steady at 15 and pH at 7.2. I am not going to actively reduce it further but keeping at filtering 75% of the water that goes in means it will tend towards 4dGH.
Two interesting asides. I mentioned in my first post the filter dropped the water to 4d. That was in a cheap canister I bought on Amazon. When I tried in the more expensive Pozzani canister it did drop it all the way to 0. Tried with the same cartridge several times to confirm so obviously not all canisters are equal. It also appears that ion exchange filters don''t gradually degrade. They work at full capacity until exhausted and then stop. So if you get a non zero reading its time to change the cartridge because the next time you use it it won't do anything.