Computer :-D My best guess on why the die off is that the microbial life is dying off some point during the grow leaving no way of chelating the nutrients into the plants. I could have started soup style, but I really wanted to play out this grow as is so that I had as much data as possible for the next grow, even if that meant losing the grow, which thankfully I haven't. It's only because I did play it out that I know what the issue is so I guess my intuition paid off in that regard. The why of it was probably the acidity in distilled water so provided I never have to use it again I should be good on subsequent grows. But that's only a hypothesis and it'll take another few grows to bear out.
It's good you have a rough idea what's going on
My Mrs. hates it when I get a new fps. Currently storming through Boarderlands the pre-sequel!
for your first full on organic grow medi! Nice harvest despite the problems with water. You have the right idea bro, learn and build on it. Best of luck with the next grow.