A few observations Andy...
So whatever poop etc within the filter etc is never cleaned out. Seems most folks clean a canister filter once a month or more. Cleaning this sucker may be a right step in the right direction.have not entered or fussed with the filter (canister) which has been the filter the whole time
If I was in your shoes I would suggest doing two water changes a week until you get the cloud figured out. Also if you do a deep clean of the substrate be aware a deep substrate can trap gases which can harm the fish if disturbed.
Pat