I've got 12 orinoco altums in a planted tank. PH 7.5, hard water, and about 20ppm nitrates. I've got 6 semi adults...I'd put them about 12-18 months old, and 6 that I would put at 6-8 months old. I have them in a 110 tall.
The 6 older angels range in size, with the largest being the size of a full grown scalare. The juvi are about silver dollar size (body).
I'm not finding them too difficult, at least not like the internet says. The came in scared (I got 6 semi adults) and sat in the importer's tank for 3 weeks. I took them home, and they weren't "right." Breathing heavy, clamped fins, but no visible signs of illness. After about 2 weeks, they broke out in fungus. Really bad, really quick. They went from fine at 9am to badly funguses at 9pm.
2 weeks of meds (kanplex in food, monocyclene in the water) and they were good to go. They've got their colour, they've calmed down, and now come to the tank when I come over.
They're very sensitive to light and shadows. It's improved since I got them, but very little. I had to wake them up over 30 mins by turning on the farthest lights in the room before I got a LED that simulates sunrise/sunset.
Happy to answer any questions. Use to run the fish room in a fish store many years ago, and have only seen these guys in the hobby 5 times. I bought them last time