A few years ago I took a 2 gallon jug of pool chlorine (double strength) and dumped 1 gallon in each of my 35 gallon SS water tanks
and then filled them with water. I left it in there 24 hours and then drained out the crud. The good news, it seemed to clean them very well. At the very least it killed anything growing in there as well as anything crawling or flying by the fill pipe.
The bad news, it took about 10 - 15 flushings of each tank to get the chlorine smell out of the water. After all I had 1 to 34 chlorine mix. For a pool, the chlorine level is measured in parts
As an aside. There was a water leak problem this past weekend in an Endeavor Cat docked next to me. The repair guy had to saw through a fiberglass
bulkhead to get to the tank and then saw the aluminum
water tank open as there was no access hatch
and no access in the compartment to inspect the bottom of the tank. He discovered that aluminum
oxide covered everything on the inside of the tank. He was there for an entire day scraping and cleaning
the inside of that tank in order to discover the leak.
I took a peek inside the tank as he was working and just shook my head
. No aluminum water tanks are in my future.