Can you give us some examples? The fact that the fishes in question grow to very different sizes suggests that they are not 'of the exact same type.' I have a suspicion about what's going o here, but I'll wait for more details. :fish:
It will depend on size of the tank, tankmates (stress levels) and water quality/nutrition. My current venestus and rostratus' I have had for about 2.5 year that I got when they were 1.2-2in. The venestus is probably done growing at 8.5-9in. The rostratus females are 8.5 and 7.5in while the males are 11 and 10 in still growing.
How big are yours and how big is the tank? Are they significantly smaller or just slightly?
I have them in a 6 foot 125 gallon tank
my female Rostratus that gave me a couple of broods was only about 5.5 to 6 inches max but it was more the height. I saw some females at a pet shop that were much longer but what caught my eye was how round and high they were almost like a small plate. The male was about 7.5 to 8 inches max
my male venustus maxed at about 7 inches, but now I have a baby that I have grown out and he is probably close to 7.5 inches and seems to be still growing
some venustus I have seen in a pet shop for sale are massive!