Can't tell if its galvanized or painted with aluminum paint from the image. Have you tried lightly sandpapering a section to see if its painted or not?
Heres a test procedure from eHow.com. It might work for you to see if its galvanized or not. Where it says wipe the solution on the steel sample, you might be able to lie the cloth across the tank. But you might still want to have a plumber or somone in the trades look at it to be sure.
Salt and water solution
Prepare a solution of three parts salt to one part water that will be used to test the steel sample. Exposing the steel sample to the salt and water solution is the quickest and easiest way to determine whether or not your sample has undergone the galvanization process.
Dip an old cloth into the salt and water (saline) solution and wipe it on the sample of steel. Set the sample aside for 24 hours to allow it to develop rust. Heavily saturate the sample in order to allow the saline solution to permeate the steel.
Examine the sample for the presence of rust after it has rested for 24 hours. Untreated steel will show the presence of rust within a day's time, while galvanized steel will not allow rust to form. The zinc coating becomes a part of the structure of the steel during galvanization and prevents corrosion.