Some scales are so stubborn that I had used hot vinegar with hard scrubbing and still wouldnt come off. My eyes were watering like crazy from the vinegar steam, not a fun way to clean a glass tank.
I also have tried using muratic acid (HCl). It works extremely well. No scrubbing at all, just use a paper towel to wip the acid on and watch it 'bubbles' the problem away. Then get another paper towel soak with water to dilute the acid. Make sure to dilute the acid concentration to 10 to 15% before use. Always add acid to water and not the other way around. Chemical gloves and goggle are required as muratic acid fume will stink the eyes and burn your skin. A face mask is also required as the same fume will burn your nostrils. It is also safe for acrylic as well. The only thing you want to avoid contact with HCl is any metal parts. Make sure you do the cleaning in a well ventilated area. I usually do it in the garage with door open. No more scrubbing or worry about cutting yourself with a razor blade, a good trade for a potential risk of a little acid burn...hehe. At 10 to 15%, it actually is really safe, just have a bucket of water next to you for washing in an unlikely chance you got some on you.
You can also prepare some baking soda water to help neutralize residual acid before setting it back up for fish.
BTW, CLR is basically sulfarmic and citric acids. Sulfarmic is sulfuric acid (H2SO4) which is more dangerous than HCl. Why pay a 12oz or 20oz bottle of CLR for $5 while muriatic acid is $10 per 2 gallon at HD and Lowes.