We have extremely hard water here, and so I have dealt with hard water deposits almost as thick as the glass on the back of my tank due to a leaky filter. The best way I have been able to take it off is to soak paper towels in white vinegar, leave them on there for 15-30 minutes, and to take a razorblade and do your best not to scratch the glass. Bowfronts are hard because they are not, well, flat. If you think about it, hard water comes from a high pH, so you need something with a low pH to dissolve it. It takes a lot of elbow grease to get it off, but it is possible.
If it doesnt come off, and you can barely see it filled, I think you should be just keep it your dirty little secret.