i have a scraper that I bought, from Petco I think, or petsmart, but long curved handle, and flat metal scraper blade.
I have found that this does the best in scraping anything off. Unfortunately, it does sometimes require a bit of elbow grease to get whatever crud it is off.
Also, I found that when a tank is medicated, for whatever reason it seems to 'bond' with algae, green stuff, or whatever, and THAT is REALLY hard to scrape off.
the razor blade suggestion is good, but I think the flat blade scraper does a good job and is a bit safer than a razor blade.