"Realisitically what school size Anostomus would you recommend in my planned tank? I was thinking 10 but could go lower or higher. I could see lemon tetra or serpae tetra as a group of 20 or 30 each."
It is really very hard to say, so I can't. I have never been able to have them in groups (3 Anostomus once, 2 Abramites once, 5 Chilodus once), but the largest tanks I have had either are 4 or 6 foot tanks. The only one that worked was a group of Chilodus, but not without some quarreling. However, all headstanders (Anostomus, Abramites, Chilodus, and many Leporinus. I can't say about Pseudanus but suspect the same) are schooling fish, leaving in shoals, sometimes fairly large. Of course they are also in and move about in large areas, huge compared to any aquaria. The exception are Chilodus, who are more chill, hanging on I small areas, sometimes not moving much. So the problem is that few people can offer the tank sizes they may need to live in groups. You seem to be planning a tank that is in that realm.
All I can say is that a large group in a suitable tank would be absolutely stunning!, and that I have twice seen large tanks like that. But I cannot know if it would work longterm.
When keeping 2-3 specimens, generally it does not work from what I see or feel IMOExperience.
It is extremely possible that individual fish (of Anostomus, Abramites) have different behaviors. However, not being able to recognize genders (I know of nobody who can), and things like that, makes it very difficult. Could it be that either females or males are gentler? Possibly but who knows and how to recognize them?
Regarding lemon tetras - I have a group of 12 in a planted 125g, shared with Hemiodus and a single flag tail Prochilodus. They stay at mid-level most of the time (photo below at left), but near the bottom during feeding. But I suspect this is dependent on the individual tank's characteristics (flow, plantings, decorations), rather than what they do in nature, which I don't know.
An awesome fish for your large tank would be Hemiodus. At least 3 species to choose from, growing large (7-8 inches in the ones shown below, sharing with lemons and flag tail), smaller on the red-tails (5-6"). Super gentle (won't eat the smallest fry), plant friendly, always active, a delight of a fish. A large tank like yours would show them beautifully.
The angels shown before. The photo is not distorted. I bought them last year as baby 'hybrid albums'. I don't believe they are that, but think they are tank bred 'Nanay altum', which is not P. altum at all, but a very nice variety of P. scalars. They are growing nicely and have now spawned twice in that weird community tank.
Again, good luck!
