Update - the CX500 Deluxe tank holds just a tiny bit short of 4 gallons... of Vinegar currently.... it's cooking as we speak. I've been convinced to attempt to clean it. (Mostly by the cost of replacing it) - I'll be running 2 filters! This thing is cruddy.
Update to the update - the White vinegar works pretty well but I went and got 4 more gallons of Apple Vinegar. I read that it's PH is double of what White Vinegar is. I'll let it soak for 4 or 5 days this time... that ought to do the trick. I also rigged up a drain auger with a brush on the end... I'll chuck that up in my drill and away the rust will go (I hope)
Inspirational photo above swiped off the internet