That's insane to owe tax on unrealized profit.
it's not really
unrealized profit though.
in the above example, you started with 10000 USD, but then you put 15000 USD worth of capital into the altcoin trade. how could you do that without realizing any profit?
i wish altcoin trades were tax exempt like-kind exchanges but the tax code says they explicitly aren't.
It basically makes it impossible to make money trading unless you immediately cash all profits out to fiat. Otherwise you could end up owing tax when BTC was at a way higher mark than you could currently cash out to pay the tax you owe.
that shouldn't
really matter because it means you are sitting on massive unrealized losses on your BTC holdings. you could offset your tax liabilities by realizing some/all of those losses in the same tax year. in fact, this may be an underlying reason why BTC crashed in december 2013 and december 2017, the last two bubbles.
best to plan for taxes on an ongoing basis throughout the tax year. if it's an afterthought at the end of the year or the following april, it can be a real headache.