My husband is taking a course on teaching right now, and this theme is something that he told me came up in the class a few weeks ago. He said something like "a good teacher is able to identify what the student is doing wrong and fix that one instance, a great teacher is able to identify the problem and teach the student how to identify the problem in the future". I profess I suck at teaching myself, I can show someone what the answer to a particular problem is, but I can't teach them the process of thinking that it takes to solve the problem themselves. And I think this is really common in general, if you think back to primary school, you're given worksheets to repeat the same type of problem over and over again so that you effectively self-teach how to resolve it, since in those cases it may not be possible to teach / we don't have enough qualified teachers to teach the thinking process involved in solving the problem at the more fundamental level.
I think that there is definitely that deeper "je ne sais quoi" about fishkeeping that a seasoned aquarist is able to identify and predict what will happen with a fish tank before overt symptoms manifest themselves, and we try to package that knowledge into rules about water parameters and temperature and what to add to a tank when etc. But I definitely agree that it is difficult to teach/learn past the rules-based knowledge to the level of being able to predict and prevent issues, and I'm not sure how that would be achieved in the aquarium hobby. With youtube, I think we are closer to that than ever since the audience can follow along firsthand as a seasoned aquarist talks through their thinking process while being "live" and showing as they go, but like with most things, the difference between being book smart and wise are the actual hours spent in practice.