Just got a used tank for our ball python who has outgrown his other, smaller tank. The "new" tank was used for fish and has that white mineral build-up on the inside. I don't want to clean it with anything that will be bad for the snake, and I also don't want to scratch the glass up.

Any suggestions on how to remove the build-up?

Try a mixture of HOT water and white vinegar 50/50 mix.