Well, lets look at it like this...its a copper tank and it smells like iodine.
What does anyone know that smells like iodine other than iodine?
I would therefore suspect someone put iodine in the tank.
What is iodine used for in a fresh water tank
Purifying drinking water
Well if someone put iodine in the water, it was probably in an effort to purify the water.
If they were trying to purify the water, must have thought that the tank needed purifying...
Now, what would make you think a tank needed purifying?
Probably either visual, smell, or taste of something in the water that didn't seem like it should be there.
If there was something in the tank that someone thought shouldn't be there,
then the solution is to clean the tank.
How do you clean a copper tank ? (the op's original question)
The answer is you soak it in chlorine and then rinse rinse rinse...,
Rinse rinse rinse rinse is equivalent to flush flush flush flush.
If u can visually inspect the inside of the tank, then by all means take a look and see if some kind of sediment is in there, if it is, get it out. If you have a small port, use a flexible pressure washer wand to break whatever is in there loose, then rinse, rinse, rinse, flush flush flush. Then probably do the chlorine thing again. That's probably as good as you're going to do without calling in a pro who better know what he's doing as some of the caustic solutions they use will screw your copper tank up bad.
Most people would probably throw up if they got a good look inside their water tanks
. You'd be amazed what accumulates in there, and what was left inside by the builders. Fuel
tanks are worse.