Sanitize the tank and plumbing, directions are in the Forum Library.
I use Clorox in much stronger amounts than recommended. I have both a Gravity and City Water Fill for my FW Tank. I put a gallon of Clorox in the Gravity Fill and fill the FW Tank thru the City Water Valve. I then disconnect the City Water and, using the FW Pump (1) fill the Water Heater with no WH power or gas, (2) run water from each Faucet until I smell the bleach (both Hot and Cold), (3) flush the commode until I smell bleach, (4) run the Washing Machine thru a cycle and then, (5) operate the WH Bypass Valve and let water flow thru it (6) let it set for a day or two.
I have disconnected my Ice Maker to increase Freezer Space so if you have one you will have to refer to your Owners Manual.
After it has set for awhile I (1) drain the FW Tank, (2) drain the HW Tank, (3) using City Water I fill and flush the FW and HW Tanks several times and then perform the above steps till I don't smell Bleach.
As bad as your tank sounds you may have to do this several times to kill the growth.
Very important to remove all inline Water Filters while sanitizing and replace with new filters when you have finished sanitizing.
