Heh, I've had similar nightmares to that, except it wasn't a tank. The shower at the house I grew up in was over the bath and it was closed in by glass sliding doors. I'd have dreams that I couldn't turn the water off, I could turn the tap so it was almost stopped but still pouring a bit. Wouldn't have been a big deal but I couldn't get the doors open and it was slowly filling with water until it got to the top and I drowned!
And the first time my wife stayed there, guess what happened? The tap broke so the water wouldn't quite turn off!