The other thing that aluminum tanks
are famous for is having through-pits right under the thickest deposit. Proceed with extreme caution. The difficulty is that aluminum
is amphoteric - it dissolves in acid or base and the cleaning
chemicals will eat the tank almost as quickly as they eat the deposits.
Also, unlike calcium deposits, which are a nuisance but generally harmless, your tank is slowly going away. Chlorine in the water
is probably a part of the problem, but I would suggest asking a water
treatment company (Culigan?) for their thoughts, since it is a local problem and they have seen it before. It probably is not required to test the water; any major city tests regularly and the results can be found on the web. Perhaps you need to consider treating the water BEFORE it goes in the tank, which may not be too onerous if you spend a lot of time tied up one place. Mixed bed
DI might do.
Good observation, John.
API Licensed tank inspector.