Finding A Fish's Age

[Yuki Rihwa]

I think to probably make an accurate guess, you'd need to dissect the fish; other than that you'd pretty much have to go on the size of the fish, assuming that water parameters were spot on as to avoid stunting

Agreed!
Also, if you research and knew the max size of your fish (if it's not at max size yet) then find the general growth rate of that species then do your math. In my experience the one that already at max size will look bulkier, thicker and you pretty much can see right out the fish face (I'm assumed that everything are in good condition like food, water...etc.)

 

[Coryloach]

I think there are ways but its rather expensive. I was reading recently about the oldest koi in the world being age tested at the time. They used its scales for testing.

 

[Lepisosteus]

From what I remember you can age fish using their ear bones or otoliths. These bones do develop growth rings that can indicate their age. Of course that would kill your fish. I feel scales would also display some type of growth rings