Reporting of ANY amount of crypto holdings is compulsory by law in Spain. So far, just holding is not taxable, but obviously if they want to know, it's because they intend to tax, eventually.
Discussion
Also obviously, the obligation to report opens the door to fining people for not doing it, or for doing it "wrong", which is also a significant source of revenue, especially now that all centralized exchanges snitch on their clients, and all bank transactions are also recorded and reported by banks.