as with most coldwater fish you cant really tell the sex of them until they mature. the male when in breeding condition has white pimples which show on his gills and around his face, often including the front fins. the female just gets noticeably plumper.
They are a neat fish! What type of set up do you have for them? Under good conditions they are quite long lived and develop personalities as well. Enjoy them....
When young the sexes are near impossible to tell apart. As they mature anal fins will differ slightly in shape but this is not 100% confirmation of sex. The best way to sex them is in breeding season when their behaviour and body shape (rounded females, slimmer males) give you a good idea, as said sexually mature males will get white pimples on the head and gill covers.
right, its been decided... they are fish of anonymous sexuality..
but now theres something else, the larger one wont leave the other alone, constantly nudging and pushing it around the tank.. it doesnt seem to be in a viscious way, but just doesn't stop..
males can do this to other fish and its irrelevant what sex the other fish is, its what a male does when he thinks theres another fish he can breed with. a lot of fish do this and as long as it doesnt get aggressive then its fine.