1. Drain the reservoir, then repair any cracks and put waterproofing (they make a trowel-on concrete like spackling) on the inside surface.
2. Install a flexible sheet plastic liner or bladder (may or may not require draining the reservoir.
3. Don't drain the reservoir but have a not-so-deep sea diver go down and patch any cracks with hydraulic cement.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.