Don't ever look inside your stainless steel water tanks. You will never never drink from them again. In this case most of the problem has to do with quality of the welded seams. In any case more was required than cleaning. And, although the water tanks were filled durring the survey to check for leaks the poor condition was not revealed.
The evidence of leaking was obvious and after cleaning the tanks sprayed water from pinholes along the seams. Also brass pipe fittings were used and galvanic corosion damaged the threaded inserts.
Regular cleaning and inspect of the water tanks could have reduced or delayed this problem. However, it seems to be a common issue. Most water tanks are plastic these days.
We decided coat the inside of the water tanks with food grade epoxy and this required even more cleaning. The brass fittings were replaced with plastic fittings. Every inside surface was primed and painted.
After the epoxy had cured the tanks were tested and no leaks could be found.