MALE VS. FEMALE FISH
How can you tell male and female fish apart? Trying to set up a single sex population can help keep spawning practices from hurting your fish, but determining males from females can sometimes be a little tricky. Depending on your fish species, there may be a few things you can look for to determine the difference.
Keep in mind that reproductive function may be limited in fish not receiving appropriate nutrition or space. When resources are scarce, fish will forgo reproductive function in order to survive. Fish in situations that are being stunted due to space or dietary restrictions are unlikely to reproduce.[...]