I am a member of a local fish club, we have a lot of different aquarium fish and would like to trade them amongst us. We all have a few fish that don't form pairs and would like to see if they pair up with someone else's. But as you can imagine keeping track of all the fish would be extremely hard and pointless if they were the same gender, so someone suggested we should buy a microscope and sex them via tissue or blood sample. But none of us has any experience doing that or if that is even possible, so we need some expert advice on the matter.
I wish to know if you can tell the gender of a fish from a tissue or blood sample and what equipment do you need (how much magnification in a microscope)? We work with discus and other cichlids, and countless marine aquarium fish.
For the sake of being specific I restrict my question to Discus fish, however, I would like to know if any general principles of tissue-sample based identification exist, for different species of fish.