Since you died the tank water, the only place clean water can come from is from the filling tube going inside the overflow. If you lift it you may not see any water comming out from it.

There is only a single valve (the float), after that valve the water split into two path: tank and filling tube. It may siphon all of the water into the filling tube, causing no raise in the tank.

You can do something: put the filling tube in the tank, see if the water raise now. If it do then you have a proof that it is that.