If you're in the US and you're involved at all with cryptocurrency, I'd highly recommend finding a financial and/or tax adviser who is familiar with what cryptocurrency is, and the tax codes around it (and mind the fact they'll probably tell you it's a risky investment (apparently people don't know this, if you look at the main board, here
)
That being said, the tax codes in the US around cryptocurrency are ambiguous at best. They could be interpreted that crytpocurrency is a commodity, as well as a security, and an income.
If it's considered a security, you're liable for taxes on *each trade* you do, as a capital gain. If you gain money, you owe capital gains taxes on that. If you lose money, you may be able to deduct the taxes on that loss. That is, IF both the gain and loss happen in the same year. You can't claim losses from 2017 against gains in 2018. This includes crytpo-crypto trading (BTC or ETH to altcoins, for example, each TIME you transfer or trade, it's a taxable event).
However, it's also considered a commodity, so you'll be liable for profit gained on cryptocurrency when you cash out from crytpocurrency into fiat money. That's considered income from a commodity, and you may have to pay taxes on that. If you use it to purchase products (like ordering a pizza) that money could be considered income from the sale of a commodity. Taxes apply.
Lastly, it's a security, so there maybe capital gains taxes that are liable when you hold the cryptocurrency for more than a year. The tax rules differ between securities you hold for less than one year, versus more than one year.
In additional to all these rules, the IRS has rules about how often you can buy and sell a security. If you bought BTC today, for example, and the price jumped to $15k next week. Then you sold, had a fat stack of cash. The *next* week, BTC crashes and is only worth $8k, but you know it's going 'back to the moon' because it always does? You may be penalized significantly for purchasing it again immediately. (they call these wash-sale rules).
Basically, get a tax adviser and financial planner who know what BTC is. It's NOT a tax haven, it's not "tax free money" and you will NOT get away with saying "the government doesn't control it, so they can't tax it" or "it's anonymous" Assume the IRS knows every trade you make.
(full disclosure, I'm not a tax adviser or financial planner, and this isn't tax advice, I've just read a LOT about this, because I value not going to jail well more than making fat stacks on BTC, Al Capone went to jail for tax fraud. Al mf'ing Capone)