Potable water
Small aircraft uses a single water tank of approx 45 US gallons where as a bigger aircraft has 2 tanks to a total of approx 90 US gallons.
The same tank supplies the galley and toilet tap or toilet flushing ( vacuum type).
The galley has a small filter which is changed at a major maintennance check.
The potable water is disinfected with chlorine at a specific scheduled check which is far in between.
The chlorination takes time. It needs a 2 hrs soaking followed by 2 to 3 completed rinsing. Every boiler must be rinsed too. It is done normally in the hangar together with a maintenance check. So it is very unlikely to be done every month. It takes a couple of 2 man hours to dump the 90 gallons water unless you want to flood the hangar floor. The whole process would take over a whole day.
So, short cut is always the way to go in chlorination of potable water. Most of the time, a simple rinsing is carried out after the aircraft is outside of hangar. That explain why the water tastes horrible after a check.
Never in my memory that I have to open a potable tank for inspection in my many years of aircraft maintenance doing numerous C and D checks.
I would not drink the potable water, same as I would not drink the tap from a hotel or cold water from house upstairs. I know what can be inside a tank. It might not make you sick but you may not be lucky every time.
My question is " will the airlines be responsible for a pax off sick after a flight with tummy ache"
Every one has his own degree of immune system. An Indian will not get sick drinking with his own country water, but you as a British will.