When you clean your heat exchangers, you don't need bright metal, just clean and free of marine
growth. Cleaners that leave bright metal are actually removing some of the tubing, soldier, etc. each time used and shortening the life. I use Rydlime. It's biodegradable and in most places can be poured over the side when finished. It can be bought on Ebay and Amazon.
I would clean the fresh water