Fill the tank with water, dump in a gallon of white vinegar, and throw in a pump to keep the water churning. Next day, the deposits inside the tank will be soft enough to wipe off. For deposits on the outside, spray with vinegar and let it sit a while. You may have to spray it several times but the vinegar will eventually eat through the deposits and you'll be able to wipe them off as well. You might want to do this outside or in the garage. I found out the hard way that wives don't like to come home to a house that smells like vinegar. And I can also tell you that the line "But honey, it's a clean smell" will not work so don't bother.