The females are a brighter yellow, and lots more red. The best indicator at even a small size for me has been the black dot on the cheek. They might not always develop the dot on the dorsal early enough. Males will be duller yellow like a washed out yellow and more blue spangling. And he will not have the black spot on the cheek ( near the edge of their gill plate )
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I have 4 salvinis 2 male and 2 female and all of them has that spot on the gill plate. The best indicator IMO is the spot on the dorsal and venting if you can.
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