Since you are talking about broader picture then let me show you the broader picture.
According to Hina she is having an upset stomach from the time she has landed in BB but I have seen her complaining only during Sapna and Hina's kitchen duties time. Reason stated - Undercooked rice.
Chalo let's say BB editors only showed it once but she has regular complains then she must be going for a doctor check up - the first conclusion which even Doctor would arrive to is change of food and water and if it continues you will not keep things lying down and even BB won't , Hina might not have seen it but BB will ask Shilpa to stop using tap water as it concerns health of their star contestant.
Secondly, I think max contestants living in the house currently are aware of usage of tap water in their food. Hiten, Akash, Puneesh learnt how to cook under Arshi who uses tap water so they must have used or atleast seen once her using it. Vikas also has seen it and none of them had any complaints about it.
The only people who must be unaware about this must be Team Hina as they just don't get out of their bed.