A water tank T of plastic material which is joined by using at least two open and identical tank parts 1 and a joining plane 2, the tank parts having respective opening structures R with a flattened inner opening edge, a sealing channel 26 being open towards the joining plane, and an outer flattened opening edge, the inner and outer opening edges within the opening edge structure are situated in each tank part 1, 35 in the joining plane 2, the opening edge at the same time defining inner and outer fitting surfaces 16, 17, respectively with a width exceeding the thickness X of the tank part wall 30 and the sealing channel wall 34 by at least about the half, such that the water tank T selectively can be manufactured from the same tank parts 1, 35 either monolithically by welding the fitting surfaces 16, 17 without sealing means in-between to each other or separable by clamping the opening edge structures R from the outer side with sealing means 33 provided between both tank parts 1, 35.