I have a guppy tank that was well established and running fine for a while. Recently, one fish started bullying another, resulting in it's death. This started a pretty up and down period for my tank (guppies got tail rot. I think the melafix used to treat it killed off a lot of the biological filter). I feel like I finally got it all under control and my parameters have stabilized, and the last two weeks everything has seemed perfect. However, on one of my guppies tonight (which is normally all black) i saw this on spot on it. It's the best picture I could get. There is another similar spot on the other side of his tale as well. The fish is acting completely normal, infact, better than he has in the last couple weeks. All other fish seem normal. Any ideas?