I'm sorry to hear about the little ones, too. I don't know that it's any immediate consolation, but in the long term, the record of your experience may make it a bit more likely that others will be able to raise their octo-babies in the future. Raising these animals is known to be challenging or harder for experts, so I think you did a great job. And as much as I like to anthropomorphize cephs because of their intelligence and personalities, their reproductive strategy is more in the "have lots of offspring, and expect that most of them won't survive" than the human style "all your eggs in one basket" approach.
There are so many variables to consider... maybe the babies have different water quality or trace mineral requirements than adults, or maybe they need some nutrition that has to come from a mixed diet or something.
Reading back, although there's some evidence of octopuses learning by watching others, at least in cuttlefish there is also evidence that the learning center of the brain develops quite a while after hatching, so baby cuttles are "on auto pilot" for the first part of their lives until the brain grows a bit, and then they start learning a bit later. This was in Hanlon & Messenger, and involved that baby S. officianalis wouldn't learn to give up attacking shrimps in glass tubes, but the adults would try a few times and learn. This seemed to be related to development of some brain lobe (the vertical lobe, I think, which is also a learning center in octos.) Which, I suppose, suggests that if mental skills scale directly with size in cephs, that giant and colossal squids might be really smart... I suspect, though, that it's mostly that tiny larvae have so little space and energy that they have to devote most of their early developmental resources to eating and growing, and don't get to thinking until later, where we go through that stage in utero... (didn't meant to hijack this thread with a science ramble, of course... I know I'm a bit weird in that a way I avoid getting too caught up in sad events is to look at them from the lens of abstract understanding, but I by no means mean to downplay the heart in favor of the head...)