In a tank B for water treatment, consisting of at least two plastic material tank parts 1, 2 produced by injection moulding or by injection moulding and embossing a separately produced separation wall T is mounted with the edge at the walls of the tank parts such that the mounting regions at least partly are watertight. The separation wall T extends in the tank B crosswise to the joining plane E. The tank parts have opening edge structures R which fit on each other and which are joined in the joining plane E in watertight fashion.