A few years ago, we closed our collection business and were faced with the problem of disposing of the 25 or so DIY 55g tanks that we had built when we started the business 10 years earlier. Over time, we had emptied the tanks of the substrates and the DIY UGF (yuk) filters, but hadn't really cleaned them up. During use the tanks were only a holding system for us, so their appearance was of little concern to us, as long as they held water and the fish were healthy.
Ten years of "deferred cosmetic maintenance" became evident when we decided to put one of the 55g's in the living room. Our 1st "real" aquarium.
Over the years, the salt spray and creep had become permanently attached to the glass surface. Since we weren't really maintenance freaks, spending a lot of time cleaning tanks, I was always under the impression that the salt could be easily cleaned up with freshwater. WRONG!!!!
Without going into a long, drawn out scientific explanation let me just say that the minerals in saltwater can form a very difficult substance to remove from glass when allowed to build up over 10 years.
Since we wanted a nice looking tank in our living room, we endeavored to make the tank as new looking as possible. Thus the challenge of removing the minerals from the glass.
I tried just about everything: scrubbing the white substance with freshwater (looked great until the glass dried), sanding the glass with fine grit wet/dry sand paper, sanding in a solution of vinegar, then ammonia, then "Calcium/Lime/Rust" remover and finally muratic acid.
From personal experience, don't use the muratic acid. It didn't work all that well, even full strength, and the stuff can be downright dangerous. Just getting a whiff of the fumes can cause serious damage to your lungs.
Getting frustrated, I looked in our Forums for some input. Reading through the postings on Cleaning Mineral Deposits, I found that I was on the right track, but I just hadn't applied the treatment long enough and I hadn't used enough elbow grease.
It seems that keeping the glass immersed in either vinegar or CLM over a period of time would dissolve the mineral deposits. Since the vinegar and CLM evaporate fairly quickly, a method of keeping the glass wet with the solvents had to be found.
Experimenting a bit, I found that putting a paper towel soaked with the solvent on the glass, sealing it with a layer of plastic (a 12" 4 mil bag in our case) and letting it soak overnight did help a lot, but it didn't leave a "just hose it off and you're done" piece of glass.
In my case is was easy, as I had disassembled the tank and had only flat pieces to work with. However, if you want to clean up a functioning tank, you can use the same method on the outside of your tank by simply taping the solvent/paper towel/plastic assembly to the upright glass with duct tape.
If you are rebuilding a tank and want to remove the minerals from all of the panes, just layer them on a flat surface with vinegar in between them. Leave overnight or as long as it takes, then scrape the minerals off with a single edged razor blade. You may find that you have to dampen the glass surface and scrape with the razor blade a few times to get the glass completely clean.
Clean the bonding surfaces with acetone and reassemble with silicone caulking.
From personal experience, the distilled vinegar ($1.89/qt) worked just as well or better than the CLR ($8.95/pt) and was less caustic to the skin.
This method of mineral removal may take a while, but vinegar is cheap and you can use the left overs in a salad dressing.