Your he is not a he at all. You've got yourself a pregnant female. She's probably going to give birth soon, so adding a chemical for treating nothing wrong was a bad idea as her fry will need to be born in good healthy water. It is also time for another water change. You should be changing the water every 2 weeks (20% of your tank - in gallons). The black spot is called a gravid spot. That is the indicator that your female is about to give birth soon. The bigger it gets the closer she is.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."