The issue may come up not that they don't wash the water tank but that the coffee pot is never cleaned, only that the old coffee is dumped. Or possibly the source of the drinking water.

I was first one to board a plane recently and as I got to the end of the gangway i could smell the fresh coffee being brewed. It smelled wondeful and i said as much to the FA. She gave a semi-scared look that I took as a warning not to drink the coffee.

Flying tomorrow morning. I am going to take one for the team and ask for a black coffee. Hoping that I can report back....