Actually, they only kill everything they come in contact with.
The problem with sterilization of *anything* is that every object has small imperfections, cracks and crevices in which the ennoying critters can hide. Some can hide in even the smallest pits and cracks.
I had the "delightful" experience of typesetting a medical
textbook on sterilization. Basically, nothing can be assumed to be completely sterilized no matter what you do. When you want to sterilize something as large as a water
tank, and when it is connected to things like water
pumps, your options are limited when compared to, say, dental implements, because water tanks
and its associated equipment
can't be subjected to everthing a piece of stainless steel
can. Then therer's the problem that the sterilizing agent can damage the surface of the object being "cleaned," leaving yet more places for the critters to hide next time.
It's not just that my water tank has been badly fouled, but that it wasn't as "clean" as most would imagine to begin with. A boat I looked at was owned by a registered nurse and her husband who was also in the medical
field. They knew my old boat had not had a water tank. They knew their boat tank had not been fouled in the way that we were discussing, but when their boat was "on the table" they warned me not to drink the water from the water tank also.
I know lots of people do it, but if one is on a longer trip and gets a significant intestinal ailment away from medical help -- it, combined with the extra physical stresses of sailing (ex: heat exposure, dehydration) -- could make them very, very sick. And, that type of illness is difficult to manage on a small boat with a small holding tank
I will never drink the water from my holding tank
after what has happened to it. If I win the lottery, I'll replace the entire water system.
This is the price
I have paid for "free" help.