How to know when juveniles have "grown out" enough for the main display tank?

[Rube_Goldfish]

I have a juvenile Corydoras sterbai that's about a half an inch long (not counting the tail fin), though I'm just eyeballing him/her. I'd like to transfer him from the fry grow-out tank he's in back in to the main display tank. That tank has a school of adult sterbais and a male/female pair of Apistogramma cacatuoides; the male is about an inch and a half (again not counting the tail fin and again eyeballing it). I know the conventional wisdom is "if it can fit in a fish's mouth it's in danger of being eaten", and I've also seen the male apisto "yawning" so I know he can open his mouth pretty wide.

So how do you know when you can safely introduce juveniles into a tank with adults? Do you have a guideline? All the compatibility guides I see assume all the fish involved are adults. Thanks in advance!

Edited March 13, 2023 by Rube_Goldfish

[Theplatymaster]

@Rube_Goldfish

 

the mouth wisdom is generally true, i followed it for my cory fry, however if you have a tank with like.

30zebra danios, and a few adult BNPs, the chances of the BNP fry getting food are low, so then its better to just leave them isolated.

ive seen the yawning in fish as well, they do this occasionally, however they dont seem to have control of this, and dont use it for hunting.

i personally wouldnt trust the apistos with the fry, just because of their large mouths, and semi aggressive personalities.