OK Tuppers, I'm feelin like we need a "reset" on this fishless cycle, especially since you want to get it done before the Cuba trip, right?, lol!
So am I right: 60L/16G fishless cycle (not sure what day we are on) non-planted?
You've had 2 pH crashes going all the way down to a pH below 5?
Have you received your KH test kit yet (API or TetraTest?)??
My feeling is that you need to do another 90% water change to maybe help get those nitrates down, and then....
I think you need to add 1 full teaspoon of baking soda (only if we are fishless here) so we can raise up what is obviously going to be a KH of zero or 1 or such. It doesn't matter, we really don't need to wait on the KH kit if you don't have it. Your pH wouldn't be crashing unless you had extremely low KH I'm pretty sure.
Hopefully you'll get the KH kit so we can follow the progress of the baking soda before we have to put more in there. You need to be posting your stats more clearly here on your thread, in a little line that includes Day X, date, time, temp, Amm, NO2, (NO3 if you do it), (KH once you've got it), Amm level after adding etc.
~~waterdrop~~