I posted this in another thread but here's something that might help...
from what I researched and in my experience, the males are more colorful and have a sharp corners towards the rear of the dorsal fin. While the female has rounded corners and a reddish belly. Here's a picture of my Rams and I photoshopped it together to outline the differences between the male and female. I found looking at the dorsal fin to be most useful when selecting GBR's and the LFS when they are stressed and or real young, in which case the reddish belly of the female would be hard to find.
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