@tilo
i'm 100% sure that, with stocks, you can compensate a losing month on the upcoming months.. but you cannot adjust in the end of the year.
in your example, you started in january winning 10k, so you will have to pay 15% (in 30 days), which is 1.5k
february you lost 10k, so you don't pay.
march you won 10k, so now you don't pay since you lost 10k in february.
etc etc etc..
in december you lose 10k, so you won't pay and next year you'll have -10k to compensate for future winning months.
but the 1.5k that you paid for january profit is lost.. and you can't do anything about it.