WIth so many coins, I think it is quite complicated to tax the the bitcoin gain:
you buy bitcoin in certain amount, then convert to another out of that amount the certain amount.
You're not talking about income from BTC trading but cryptocurrency trading in general. Why did you put it in bitcoin discussion when you're asking about altcoins and bitcoin as a whole, not to mention there's a legal section for this?
So here is my plan for taxation purpose to make it as simply as possible.
To make it as simple as possible when your tax year comes to an end you sum up all your cryptocurrencies and check their value in fiat currency that you're using to pay the taxes. Next you subtract it from the money you've used to buy those currencies, or the amount that you held when you've submitted your report for the previous year.
But first there is one question? Does tax incur as a result of buy/sell or move in and out the cash (not the coin) out of exchange.
I'm trading in the Euro zone and most countries here only require you to submit yearly reports on your gains, which means that you're counting the money you've wired from exchanges into your bank accounts.