hcl and water. It is the cheapest most effective method. Do it outside with a mask and gloves. Put the tank on its side and pour enough water in it to cover the side on rhe ground then add a few ounces of hcl. Wait like 15 min Rinse it out with a garden hose on low pressure to avoid splashing. Repeat for the other sides. If it doesn't come clean increase the acid concentration. Can be purchased at your local hardware store in pool care.
Hard water deposits are a normal fact of life on almost everything that comes into contact with water where I live. Most of the time, white vinegar and some patience will take care of it. Use some folded paper towels, or sponges, or washclothes -- something that you can drench with the vinegar & then press it tight onto the hard water gunk & let it sit for a good while, probably at least a few hours, or better yet overnight - try to make sure it will stay wet. Or just pour vinegar straight on the side, if the lip will let enough liquid sit there. I usually have better luck with putting a cloth/towel up against it myself. Letting it soak like that usually makes it come off a lot easier than trying to scrub at it immediately. You might still need to rub a bit, but it should be loosened up at least somewhat from soaking. The acid mentioned above will work too - as will a $1.00 bottle of "The Works" toilet cleaner from Dollar General (which is primarily hydrochloric acid too), but I usually use vinegar on everything possible, and only resort to acids as a last-ditch resort, since I'm kinda wimpy about messing with that harsh chemicals. On a few oddball items, lemon juice has actually worked better for me than vinegar - but usually vinegar does the job.