Although polythene water tanks are very strong, they occasionally develop leaks from harsh weather conditions or rough working environments. These tanks can be very expensive to replace. If your poly water tank has a leak, you can easily fix the hole with a heat gun and a few minutes of your time.
Drain the tank. In order to repair the tank properly, you'll have to drain the tank below the level of the crack before starting the repair process.
Clean the area of the tank that needs to be repaired using soap and water. The surface of the tank needs to be free of any oil, grime or paint--just pure plastic.
Drill a hole at each end of the crack with a small drill bit. This will prevent the crack from extending any further in either direction.
Preheat the repair area with a heat gun or propane torch. Heat a slightly larger area than you are repairing to make sure that you have an even heat distribution over the entire crack.
Add the filler material. Using a small strip of polythene filler material, place it over the crack and continue to heat the repair area with the heat gun. Slowly move the material so that it fills the crack completely. Smooth out the molten plastic with a putty knife so that is fills the entire area. Trim off any excess with a utility knife.
Test the tank. Once the repair is cool to the touch, refill the tank with water and check for any leaks. If a small leak appears, repeat the repair process.