Salt water aquariums get splashes or spray of salt on the glass and equipment used in the aquarium and this "salt creep" can become permanently attached to the surfaces of your saltwater aquarium. Unfortunately, the deposits are not just salt (sodium chloride) that is easily washed away. The minerals in saltwater can form a very hard substance that is difficult to remove from the glass when allowed to build up. While you may have ignored this for a while, it can become an issue when you want to remove it. Now you have to face the challenge of scraping the minerals from the glass.
You might have tried some ineffective methods of mineral removal. Simply scrubbing at the white substance with fresh water will look great until the glass dries and then you see that the buildup is still there. Sanding the glass with fine grit, wet or dry sandpaper also will not do the trick.
Various solutions such as vinegar, ammonia, calcium/lime/rust (CLR) remover, or muriatic acid may be effective. But use with caution, as muriatic acid can be downright dangerous. Just getting a whiff of the fumes can cause serious damage to your lungs.
Vinegar or CLR
You can safely use vinegar or CLR to remove the salt creep, but the important factors are applying the treatment long enough and using enough elbow grease. Keeping the glass immersed in either vinegar or CLR over a period of time will dissolve the mineral deposits. Since the vinegar and CLR evaporate fairly quickly, you need a method of keeping the glass wet with the solvents.
Put a paper towel soaked with the solvent on the glass, sealing it with a layer of plastic (such as clinging plastic wrap used in the kitchen) and let it soak overnight. This will help with many deposits, but you may need even longer treatment with a thick deposit.
It can be easier to treat a disassembled tank because you have only flat pieces of glass to work with. 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 remaining 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 the glass with silicone caulking.
If you want to clean up a functioning aquarium, 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. Then, scrape the glass with the razor blade to remove any residual minerals. Rinse the glass thoroughly with fresh water to finish.
Distilled vinegar works just as well or better than the CLR and is less caustic to the skin. Vinegar is far cheaper than CLR. It may take a while to dissolve the minerals, but you can find many other uses for the leftover vinegar around the home. Not only is it useful for cleaning many surfaces, you can, of course, use it for making salad dressing.
Medical Management Guidelines for Hydrogen Chloride. Agency For Toxic Substances and Disease Registry, CDC.