I have a rainbow 320 chlorinator that, over time, seems to be putting out less and less chlorine.

When the pool was new, it'd digest about 3 or 4 tablets a week at a setting of 2.5 to 3. Now it takes about 3 weeks to digest that much, at a setting of 5, and I wind up having to constantly add Hypo to bring chlorine up.

The pool was built in '99.

Any ideas what the issue could be?