I've seen this problem several times. First of all, stray current DC corrosion inside an integral tank on a metal hull is virtually impossible. This is most likely a type of galvanic corrosion called "deposit corrosion".
When water containing copper ions is introduced into an aluminum tank, the copper, being more stable than the aluminum, plates out in metallic form on the surface of the aluminum. The copper in this case comes from trace amounts in water entering the tank, picked up from the copper pipes through which it runs. The amount of copper in each filling of the tank is miniscule, but most of the copper that goes into the tank stays there, especially on a boat, where the motion prevents stratification and tends to bring most of the water into contact with the tank wall at some point during its stay in the tank. Eventually, there's enough copper to cause galvanic corrosion of the aluminum, in the form of pitting.
This problem is actually worse in fresh water than in salt water - in sea water the galvanic effect of the small copper deposits is distributed, while in fresh water, due to lower conductivity, while the total amount of galvanic corrosion is less, the effect is more concentrated around each deposit, causing pitting.
A similar problem occurs in bilges of aluminum boats where stranded wire fragments from wire trimming during electrical work, non-ferrous fastenings, or coins are dropped in the bilges.
You can prevent it getting worse by deionizing the water that goes into the tank, but only abrasion will remove the deposited copper. Coating the tank after the abrasion may help. In extreme cases, you may have to replace severely pitted areas of the tank walls, then insert a liner, typically plastic, spaced away from the original tank walls to allow the original inside surfaces of the tank to remain dry.
Anodes would help prevent the corrosion due to the deposits (but only well below the splash zone in the tank), however, in order to work, the anodes must dissolve, and both zinc and aluminum (and aluminum anodes also contain zinc and magnesium) are considered toxic in the right amounts. Magnesium anodes are used in residential hot-water heaters, but the assumption there is that people won't be ingesting much of the water from the hot water line.
Good luck
Ed McClave
