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 that is a fact that every one has to pay tax on fiat currency, but it we talik about bitcoin so there is no tax on bitcoin because bitcoin is not a legal currency, and still not legalize by any country.