Is the water tank hidden in the wall. Professionally speaking, this is the core component of the hidden bathroom installation system. As a part of the water supply and drainage system, the water tank is hidden in the wall. Hidden water tank is a relatively advanced design concept. It is said that it was first developed by Geberit, one of the best water parts manufacturers in the world. Now it is very common in Europe and America, and it is also the standard of mainstream toilets.