
Claire, we aren't trying to rain on your parade. The advice being given here is in the interest of trying to make that fish as happy and healthy as it can be.
If you get this hex tank and if you still bring home this altum, just keep him in there alone. Do not worry about pairs and trios and such. He will not be "lonely." And he will need all that space for himself.
Should you look into a larger tank in the future, a 120 would be better than a 125. A 120 is deeper, the 125 is longer.
Sylvia, Where did you get the information that Pterophyllum altum amd Pterophyllum scalare are the same species. This is not true. Wild P. scalare are silver with black stripes. They have a slight indentation of the snout and sometimes almost none. There is a subspecies of P.scalare from deeper waters in the Peruvian areas of the Amazon which are higher in body that other varieties and are sometimes mistaken for YOUNG P. altum.
P. altum have brown stripes rather than black and a very distinct indentation of the snout. They have been recorded in deep water river dives as having been 18 and even 20 unches tall. P. scalare does not reach this size.
Ichtheologists also count rows of scales and number of scales at the lateral line row. But I do not feel like looking these figures up.
I have 3 full grown adult P. scalare (1 gold, 1 koi, and 1 silver, not that color matters) and I have seen 2 full grown P. altums at the zoo in Memphis. These are not the same species.
Claire, Think long and hard about how much work you want to put into this fish before you bring him home. Only do it if you are going to do it right. And if you are, then do.