Just plain ol regular household bleach. I use 1/2 cup in a couple of gallons of water for my 33 gallon tank and 6 gallon water heater. Suck the bleach solution into the tank, fill up the tank, then run the pump to fill the water heater and other lines. Let set for a couple of hours, then flush a couple of times to get the chlorine out.
2015 Greyhawk 29MV
Honda CRV toad with Blue Ox baseplate
Ready Brute Elite towbar with braking system
TST 507 Flowthru TPMS