Abstract: An underground water retention or detention system for collecting stormwater, rainwater, sewerage, effluent or other waters, and controlled release of the water is disclosed. Typically, the tanks are formed from a plastics or polymeric material such as high density polyethylene. The pipes may be polymer coated corrugated steel pipes. Tanks suitable for use in the water retention and detention systems are also described together with connection fittings for connection of pipes to the tanks. Generally, the connection fittings are adapted to provide structural support to perimeter walls of the tanks about respective openings formed in the tank for the connection of the pipes.