ok so i was clearly not 100% factual on my response what i should have said is hey you can vent fish, you can tell sex by finnage in some species and you can tell by coloration in some species, but with in mind there wasnt any certain species described by the thread starter so this could intel alot of species but as a general rule you can vent a fish with a magnifying glass for a guarantee on what sex you have if thats what the person who needs to know really wants to do, as for me i dont like to stress my fish in that manner so i go with the old age rule of buying six specimens and let whoever pairs up sort them out and then i remove the excess or not its my call, my hobby............peace