Stanz advice is on point, however, don't expect a Dicrozoster or an Abalios Geo to get 12-15", they are typically gonna push 8-9" and stop. Altifrons can get on the larger side, but for the most part these guys typically stay a few inches under 10.
that little female Geo I have been passing around half-assed pix of, looking for an ID is well over 1 year, (6 months in my care) and she's BARELY 4"Agreed.I have yet to see any of the above reach more than 8 inches.Still a sizable fish.6 adults will fill the op's tank nicely.