I recently cleaned out a 10g I found in my parents garage (bonus!). It
had some scum on it. Tried vinegar... worked ok. I then just took a
razor blade and scraped the glass. I can tell where I missed because it
worked very well. This may not work on mineral buildup due to
scratching, but I was fortunate not to leave and knicks or scratches.
Nothing beats the blade. If you try the blade on an empty tank, try the
bottom first as it will most likely be covered in substrate in case you
need to hide scratches.