If you convert your bitcoin to the fiat and then you should pay the tax since you are literally making profit by convert your bitcoin to fiat, however it's depend on which country you live due to rules and regulation of each country is different. Unless you don't need to pay tax when you hold your bitcoin on your wallet or transaction with them.
yes there is no tax on bitcoin directly but if you change your bitcoin in to fiat currency and deposit it in bank they you have to pay tax, actually bitcoin is not yet consider as legal in any country therefore there is no tax on bitcoin any where.