My experience is that some species takes much longer to mature. P. parvulus is one that takes the longest in my experience, whereas a species such as p. nagyi seems to mature quicker.
However I am also not sure if other things may influence the growth, I have no real observations.
My paros share mostly the excact same conditions – except sometimes the amount of fish in one tank.
The sort of very non-scientific observations that one does in a private setting are interesting but very unprecise. For instance, I would say my ‘practise’ now with p.parvulus is that I would avoid taking young fry from the ‘birth tank’ too soon, EVEN they look quite big, – and I would not set them in a tank with few fish. I think my experience has taught me that the fry I leave in the tank, even when there are many grow quicker and more healthy that the fry I take out and seperate in a small flock.
I separate because I think that it is better for fry to have plenty of space etc – but the fry I have done this with have not developed as good as the ones that are still in a kind of ‘flock’.
In the same kind of ‘manner’ (very uncertain observation) I would also now avoid if possible to have one paro alone in a tank. It seems to be my experience that having a lonely paro equals more likelyhood of illness. This makes me also wonder about what we really know about the nature of paros in not being a flock fish. I know they are not, but to me it seems the flock has some importance in their wellbeing.
Anyway 🙂 .. just my two cents.