what is that spot inside my fish by its tail under its stomach?

[whisper2000]

i have a small silver fish with a lil red tail and i have two of them ....you can see inside of the body and see the gutts and stomach but one of them has a dark small circle thing like a lump inside of it and its under the gutts but near the tail ...what is that?? and how can i determind the sex of my fishes lol..???