Hey guys ! I have a some guppies in a tank and they all were very active and playful. Recently one of them has been hanging out near the top of the tank and acting more slow ?? This fish has always had the black and orange coloring. I've had this tank for about two weeks. There had been one death just a few days ago and I did a 50 percent water change. There are 8 fish in total, 10 gallons. It just won't stop skimming the top of the tank and none of the other fish do this?? What is wrong??