In my opinion you should start tracking your winnings and buy-ins when you come above the tax excempt amount ("Steuerfreibetrag") with your winnings, when you are playing on a regular basis. And I am pretty sure, that the tax offices will consider this as a regular basis, when you are playing nearly daily some tourneys, what many of us do here already by playing the FRC games.
I am not sure anymore, if it was 15 or 20 hours per week. But if you exceed this amount of time in one week, which is easily done by MTTs, then it can be considered as a subsidiary occupation ("Nebengewerbe"). And you should add winnings of this to your annual tax declaration, when they are above your tax excempt amount ("Steuerfreibetrag") to be safe not being accused of tax fraud.
Usually, I am tracking my results with PT4 already, but unfortunately it also is not always reliable and therefore I am considering to do it additionally with some excel sheet or so. Well, of course you could also add your buy-ins then to the tax declaration.
