To clean a plastic water tank, open the outlet valve or tap and let the water flow out. Next, mix hot water with laundry detergent and use a bristle brush to scrub the inside of the tank with the cleaning solution to remove slime and sludge. Then, rinse out the tank thoroughly and flush the hoses and pipes. Finally, fill your tank with a mixture of clean water and bleach and let it sit for 24 hours to disinfect it. For tips on using a pressure washer to clean out your tank, read on!
Source: Wiki How