Keeping your tank clean removes the habitat that bacteria, protozoa and even viruses can use to get a foothold in your water system.
What is the top reason to clean your tank?
Microorganisms can use the sediment that builds up on the floor of almost all potable water storage tanks and towers to get a foothold in the water system. Hidden from treatment chemicals under the protection of the sediment these contaminants can grow until they deplete chlorine reserves and become a public health problem.
Microorganisms causing waterborne diseases include protozoa and bacteria, many of which are intestinal parasites, or invade the tissues or circulatory system through walls of the digestive tract. Various other waterborne diseases are caused by viruses. (In spite of philosophical difficulties associated with defining viruses as “organisms”, it is practical and convenient to regard them as microorganisms in this context.)
Removing the sediment removes the habitat that allows these microbes to remain hidden in your water system and thrive.
I found a great article on contaminants found in potable water check this out.
In Potable Water by Jeremiah Castelo