First, these are the Spanish lawyers who know the most about cryptos taxes. I may go to ask them in the future.
https://www.abanlex.com/areas-de-practica/derecho-de-internet/bitcoin-y-otras-criptomonedas/As for what you are asking for Hacienda (the Spanish law department): investing in cryptos is like investing in forex, you only pay taxes on them when you convert them into Euros, because you may lose everything. There are some exceptions to this like the Impuesto del Patrimonio, but as far as what you are asking I think it will be enough to declare them when you convert them into euros.
That quote you put is the opinion of a lawyer, he says that you should pay taxes when you exchange cryptos for btc or the other way around, but Hacienda hasnt means to check that nowadays.
This taxation is like when you buy and sell stocks or funds. You may have bought them at different times and prices, so if you sell part of your investment, the FIFO system applies.
If you are taking about a significant amount of money, it is worth spending a little bit asking abanlex lawyers. For small amounts, if you pay taxes, Hacienda is not likely to mess with you.
This is the kind of clarification I was looking for!
I got a feeling the lawyer was talking about just his own opinion, but wasn't sure.
Having to do individual profit/loss calculation on each trade would be a monumental task to do and if it ever got to the point where a tax office would ask me the details, I'd perhaps print out several hundreds of pages of exchange transactions and let them figure it out themselves

To me it sounds most plausible that they won't bother if your income is not astronomical.
Thanks a lot for referring that firm, I've been looking for ones if I ever need some consultation in the future!
