[unclebilly]

Much of that deposit is bacteria based. There are 2 types of bacteria, planctonic and sessile.

Planctonic bacteria float around in water and is fairly easy to kill with bleach, biocides, oxidizers, etc.

Sessile bacteria attached to surfaces and is very difficult to kill or remove by flushing a tank or using bleach… you need to mechanically remove it. These colonies are called biofilms and remarkably the bacteria work together to create environments suitable to each species.

Don’t believe me?

Do you brush your teeth or just use mouthwash? Plaque is exactly this, a biofilm and tooth decay is the result of bacteria. Ever 6-12 months, you get a dental hygienist to scrape off the biofilm that you can’t get with regular brushing.

Another data point… swimming pools are loaded with chlorine- way more than we use to kill planctonic bacteria in industrial applications. Ever notice that the walls of the pools are ‘slimy’? That’s the biofilm. The filters are also loaded with that nasty biofilm that thrives because of the flux of bacterial fool sources.

You can’t clean a hot water heater. Flushing them accomplishes little.