A method for assembling a tank, including: providing a plastic first shell, a plastic second shell with an opening, and at least one connecting member with a plastic end part configured to be arranged in or around the opening, the first and second shell delimiting an internal volume of the tank, and the connecting member being dimensioned for connecting the first and second shell in the internal volume of the tank; welding a circumference of the first shell to a circumference of the second shell; arranging the end part in or around the opening; and connecting the end part of the connecting member to a wall part of the second shell by heating an external area of the second shell surrounding the opening and/or by heating the end part of the connecting member.