Performed a tattoo(hard water stain) removal on my 50gal breeder plant tank. Like most of my tanks it's been empty since I got crushed by the big snow storm(thanks Pepco)
The glass had some really tough deposits from me leaving it !/4 full and letting the remaining water just evaporate out, I tried vinegar full strength that didn't work. Next I tried CLR Cleaner(Calcium,Lime Rust) full strength and let it sit a little while. This worked some but the stains seemed like they were etched in the glass.
Finally I remembered my mentor George had told me once about using Muriatic Acid as a last ditch effort. Although I would not recommend this method because its probably cheaper and safer to just buy a new tank(Gloves,mask,googles,Acid around$60) since I already had the proper equipment I took everything outside tried it.
It took a couple times but it worked. Like I said before I wouldn't recommend this method because it has to be done outside and you need a bunch of safety equipment