I think 9 is an ideal number for which to strive. It's not always possible though, for one reason or another. And there is some wiggle room with different types of fish. For example, I've recently found out that Angelicus loaches and zebra loaches hang out together. I have 4 beautiful Angelicus, but they are wild caught and have not been in season, and the ones that have been available are not good enough to bring home. I added the zebra loaches to the tank not too long ago 8-10 of them. Well, they all live together in and around a tree stump decor. I feel better about only having 4 of them because they are often with the zebras. I don't think they are WITH the zebras - they definitely maintain their group hierarchy, but I think they're happy to have the zebras in there. Rainbows are another. Different species within the same genus will school together from time to time - otherwise they are with their own. For this reason, I don't think it's as necessary to maintain such large individual groups. 2 schools of 5 almost make a school of 10, is what I mean. My goal is to have 20-24 rainbows in the 125 - 5 or 6 of each kind.
However, I haven't found different species of barbs to school together ever. Or tetras, or Corys, though in my experience a lone Cory will join up with a group of others. But different groups of Corys all did their own thing in the 90.
There is no black and white - only grey. It's just that some of us see our particular shade quite clearly
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