I had the same problem with my old set up. Salt mostly for me. If you have a filter output that cascades into the tank you are getting splash. Airstones in UGF, bubble walls etc also cause splash. The water hits the lid, then evaporates. Leaving the minerals behind.
My new set up has a custom hood/cabinet enclosing the top. No lid covering the top. Not much for the water to splash on. I'm sure I'll have some.
They don't list crud scraper as one of the things you need for keeping fish! But you do.