Stubborn stuff on glass help!

[slimmacck]

Hey everyone! Im a returning hobbyist after about a 9 year hiatus. I picked up a 28 g nano cube off Craigslist a few weeks ago. I scrubbed and rinsed it and set it up. I didn’t notice until after the cloudiness disappeared that there is some sort of calcium deposit all over the glass from the previous owner. I have scrubbed and scraped to no avail. Any ideas of what this is / how to treat it? The tank is still newly established and not much in terms of life is going on in there so I can remove the water but I’d prefer not to if possible. I read possibly white vinegar and drained half the water and scrubbed a test area with it but the results were quite minimal. I’m running live rock + sand also.

 

[Saltyreef]

Razor blade if its glass.
No need to empty. Just scrape away at a sharp angle.