How do I clean out a Super Store water tank ?

I'm not sure, but here's a link where you should be able to contact someone in the company:

http://www.htproducts.com/superstor-ultra-waterheater.html

Wow you are lucky to have it 20 years. The new ones don't last nearly that long. See no reason why flushing should be different.

Turn the Knob on Your Hot Water Heater's Thermostat to “Off” ...

Turn Off Gas to Hot Water Heater. ...

Turn Off the Cold Water Supply to Hot Water Heater. ...

Turn on the Hot Water in a Sink or Tub. ...

Open the Pressure Relief Valve. ...

Connect Garden Hose to Drainage Spigot. ...

Turn on Spigot and Drain. ...

Flush.

this should help https://www.wikihow.com/Clean-a-Plastic-Water-Tank

Not sure if this will help you, but hopefully! https://www.doityourself.com/forum/boilers-home-heating-steam-hot-water-systems/545346-flushing-my-indirect-water-heater.html

see if this helps you out https://www.youtube.com/watch?v=zmkCvuaLtZE

HOW TO CLEAN YOUR WATER TANKS

https://rainwatertanksdirect.com.au/blogs/how-to-clean-your-water-tanks/

If your superstore tank is electric, here's what you do:

https://www.thisoldhouse.com/plumbing/21016872/draining-flushing-an-electric-water-heater

Hi, here's a lovely article that will help you out - http://www.community.gov.yk.ca/pdf/cleaning_disinfecting_water_holding_tank.pdf

https://www.wikihow.com/Clean-a-Plastic-Water-Tank

I think this post can help you also:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=26&cad=rja&uact=8&ved=2ahUKEwjj6pXPh6noAhUXCs0KHQFcDaMQFjAZegQIAhAB&url=https%3A%2F%2Fwww.waterheaterhub.com%2Fhow-to-clean-a-water-heater%2F&usg=AOvVaw23j84_UPot0OKaz9VkgtoA

Drink it all. Good for skin

Hey, i would like to share my failed attempt to clean my water tank. I cleaned it with bleach and it ruined it. the issue with galvanized tanks is leaching of zinc. And that layer of zinc melted after cleaning it with bleach, This happens more quickly with warm acidic water after some years. The people that have success with galvanized tanks seem to always have hard alkaline water or use the tanks for winter but with hot water or bleach this leaching will damaged easily, i searched for best budget tanks company in 2020, i found a brazilian company which developed new silver layer for tanks.

Thanks

You should be able to hook up a hose to the water valve. To flush your hot water tank, simply turn on the cold water spigot leading into your hot water tank. Let it run for a few minutes until the water exiting your hose runs clear. ... Continue flushing until you have very little or no sediment in your water