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.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.