Hello again,
It could just be bad luck... :ske:
The idea of him being too young to be sold is not about separating him from his siblings, what I had in mind was thee chance of survival in varying water conditions.
Hardiness increases with time to a certain point because when the babies near 3 months of age the fish ( fins, scales, body, colour as well as breathing and immune systems) is still growing and coming to be fully operational. There are other conditions such as feeding and care that determine development.
So what I'm saying is... at two months of age it is not a fully developed fish yet.
Its kind of like saying a 5ft6 80kg grammar school first 15 rugby team member is already an all black.
Just wont have the experience or skills to survive on the field... for long.
I'm just saying that as a comparison (I know its not that similar) .
Here its not the same but similar as the fish would probably not be ready to adapt to different water conditions or have an immune system to produce antibodies to whatever may have been dormant in the breeders water, whatever it was probably became active in your waters and attacked the most eligible host because the conditions may have been right for it to come out of dormancy.
I have before dropped in my adult fighters into water which was 17 degrees and quickly put the heater on, they have survived until the water caught up to required temperature (to be fair they were plakats and I was doing a study on adaptation and sensory usage under ideal temperature).
But young fry can die at 23 degrees.
Personally I think you should take it in as experience and get a bigger fish next time, only keep it to acclimate until both container water and tank water are at same temperature so max would be like an hour?