Know what the structure will hold
A drinking-water tank, a fire-water tank and a swimming pool each have different requirements. For potable water, the waterproofing system must be approved for contact with drinking water. For pools, the system must suit the tiles and adhesives that will be installed on top.
Details first
Most tank and pool leaks start at the same places: pipe sleeves, construction joints, corners and wall-to-floor junctions. These are treated first, with fillets, reinforcement or injection where required, before the main coats are applied.
Curing and testing
Cementitious and other systems need time to cure before water is introduced. A water-retention test, filling the structure and monitoring the level over an agreed period, should be carried out before tiles or finishes go on. It is far easier to repair a leak before the finishes are installed.
Access and safety
Tanks are confined spaces. Ventilation, gas testing where required, a standby person and a rescue plan are part of a safe method statement, and should be in place before anyone enters.