Are you an European Union citizen or resident?
The EU isn't one country with one tax system, it's all different countries with different taxes.Quote
If yes, when are you declaring your cryptocurrency profit?
I can only speak for the country I live in, and split it up my tax declarations in two categories:-Trading or increase in value: never! Trading on a non-commercial basis isn't taxed, there is no capital gain tax. We have an annual tax based on the value of everything we own though.
-Earnings: always
For income tax I declare the euro value the moment I earn it.Quote
when do you this declaration?
Taxes are filed once per year here.Note that I'm talking for consumers only, if you're a company, or earn Bitcoin from mining, it gets more complicated.
Quote
If 3), did the tax office knocked your door to ask about the unusual deposits in your bank account?
This is the exact reason why you shouldn't try to hide your crypto from taxes. If it's ever worth enough to buy something big, you want to be able to do so (without being treated as a drug dealer).My question is related only to the profit you can make from converting Bitcoin to Euro, after you bought it for a cheaper price. Easy example: you buy 1 BTC for 1K EUR, you sell it six months later for 10K EUR. You made a profit of 9K EUR. I suppose you need to pay a tax for that 9K EUR profit.
No, I don't 
It would help for your question if you say which country you live in.