Internet currencies like Bitcoins are getting common these days. The market is highly volatile. Profits as income from private sales trading Bitcoins are not liable for income tax if the period of time between acquisition and disposal is longer than one year. If you do various transactions the FIFO method (first-in, first-out) applies, the Bitcoins bought first are considered to be sold first. If there is more than a year between acquisition and disposal, the speculative gains are tax-free.
Losses from private disposal transactions can be offset only against speculative profits from private transactions. Speculative losses emerging after more than one year will not reduce your income tax.
If you hold a loss position of Bitcoins, sell it just before you hold it for a year and buy again promptly afterwards. Thus you are able to secure the tax benefit from the loss carryforwards.