Different things work for different people, but here's my two cents.
I use 1:3 white vinegar/water solution in a new, unused spray bottle. You can also use it full strength of removal of deposits if they're really tough. The spray bottle lets me get the sides, etc. If its a really tough job- Instead of tilting the tank or sitting it on end, you could dampen plain white paper towels (no dyes) or cotton towels with the vinegar or vinegar solution, then drape those over the sides to sit against the glass. This will allow the vinegar solution to stay in contact with the glass and stay wetter longer. You can use the spray bottle to re-dampen as needed.
As mentioned, you can use the scrubbers sold for aquariums at pet stores, or a razor blade scrapper. You can use plain white paper towels. You can use cloth towels, but I send mine through the washer with only hot water and the dryer with no dryer sheets. Then I feel comfortable that there's no chemicals, etc on the towels. I also use dish washing clothes that have one side of scrubbing material. Know what I'm talking about? The microfiber ones are what I like best. I use the same laundering process on those as regular towels. I do NOT use regular household sponges! These are often treated with chemicals/cleaners!
And, as everybody else said, RINSE THE DAYLIGHTS OUT OF IT!
So I wish you the best on this. I loath cleaning up even mild deposits, so I feel for ya! But it will be sooooooo worth it. Good luck!