I recently purchased a very used 120 tank locally where the water is extremely hard like 800 PPM Cal/Mag. I almost didn’t buy the tank as it had hard water level marks, hard water haze and hard water damage in some spots. I tried everything from hanging paper towels soaked with vinegar and plastic rap for days, CLR, cleaners, razor blades, corn starch with a drill gun polisher wheel. Nothing, and I mean nothing worked that wouldn’t take months to clean at the rate of progress I was getting and I was ready to just give up and ignore the haze. Until I found an obscure article about someone who removes deep scratches from aquarium glass. The method used to remove the scratches isn’t relevant here or needed but the method used to polish the glass back from a scuffed to a clear finish after repair is relevant here and worked wonders. Turtle Wax Heavy Duty Rubbing Compound! Not light or medium duty but, Turtle Wax Heavy Duty Rubbing Compound! It takes it all off 50 times faster then anything else I tried. I mean dap some on a polishing pad (terrycloth with sponge inside) rub a spot the size of your hand in circles or linier it doesn’t matter, for 1 minute and the calcium, marks, damage is all gone in that spot and the glass is bright and showroom clear. After your done on the inside clean up with paper towels and vinegar to remove the volcanic pomus and silica powder residue. I used it and it made the aquarium glass shine like showroom new and my fish after 2 months are all just fine. Unless someone can show me something works better I will never ever use anything else again.