Technically you’re supposed to pay taxes on every transaction. So moving from BTC to ETH your supposed to pay taxes on the gains/losses of your original BTC from when you bought to when you sold for ETH.
And then when you use your ETH to buy CND (and you should ;) you are supposed to pay taxes on the gain/loss of your ETH while you held it.
RE: AVOIDING INCOME TAX (15%avg) when cashing out your BITCOIN into FIAT (theoretically of course!)