We have a pretty new water heater, and an old water softener that no longer takes much iron out of the water. We have a new softener on the way, and wanted to clean up the water heater if that's possible.
We have products like Iron Out and Whink Rust Stain Remover that work great on toilets and stuff, but we don't know how or if we could get it into the water heater...
If a thin layer of iron compound happens to form on the inside of a hot water tank, you would need a large quantity of C.L.R. or similar chemical to go with the 40 or so gallons of water in the tank. Turn off the heat. Drain the tank about 1/3 of the way. Dissolve the chemical in some water first, then pour it in, refill the tank to the brim, and stir with a stick. (take off the hot water nipple and put a funnel in). Let the mixture sit there for several hours with stirring again every now and then.
Drain and refill the tank completely twice before turning the heat back on.
The above is cumbersome enough to not bother getting the iron out.
(added later) I would use no more than one third the amount of chemical normally needed, for example if one tablespoon goes with one gallon of water, use no more than 13 tablespoons for a 40 gallon tank. There is almost always some exposed steel in the tank after just a few years of life and you don't want the chemical to eat away at tank material or components too quickly.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.