I was going to ask some of the same questions, but I figured there must be a good reason he is using 100% RO
water as it isn't easy/cheap to buy/make.
Yeah, a downside of RO
is that there is no buffering capacity, but that can easily be increased by adding Calcium Bicarbonate. And, like you said, there are no metals or nutrients present in the water which help both the fish and plants.