Delivered water should be potable (safe for human consumption) and obtained from an approved source. It is necessary to clean and disinfect your water holding tank(s) at least once a year or more often, if required. This is to remove algae (plant growth which produces bad tastes and odors), silt, and bacteria which may be harmful.
Cleaning your own Water holding Tank is dangerous and should not be attempted by anyone who is not experienced since it allows you work on a confined spaces , leave it to STS. We assure you it will be done for a couple of hours.