Not only ACJ, but all A319/20/21 have an ACT option.
The Qatar Airways A319LR have provision for six tanks, but operate with only one fitted.
The charter operators in Sweden have them on their A321s as well.
There is now a new option for the A318 as well. It is different because the A318 cargo doors are smaller.
BA was talking about fitting an ACT in their A318s this summer, but haven't heard anything lately?