If you buy a (large enough) amount of some currency; later sell it, and the difference makes you a profit - then it's taxable income.
Intent doesn't matter as such - if you do it for 'practical purposes', then you most likely don't realize the gains (don't trade USD->EUR->USD, but do USD->EUR->buy stuff in Europe) or have very small volume (lots USD->lots EUR->go on trip->convert a bit EUR back to USD).
Intent doesn't matter as such - if you do it for 'practical purposes', then you most likely don't realize the gains (don't trade USD->EUR->USD, but do USD->EUR->buy stuff in Europe) or have very small volume (lots USD->lots EUR->go on trip->convert a bit EUR back to USD).