I have had four neon tetras for almost a year now and one died yesterday after it developed a black coloration between its dorsal fin and its eyes. The coloration developed for a week or two and then I noticed it starting to list aimlessly and shimmy a little back and forth. It died the next morning. The other three appear to be normal. I also have an albino cory catfish, a Plecostomus catfish, and four fancy tail guppies in the tank as well. It is a ten-gallon set-up with a heater and a Tetra Whisper Filtration system. I am worried about the other fish in the tank since the one Tetra that turned black died. I have researched fish diseases on the web and have not come up with anything. Can anyone help me or suggest anything so that I do not lose my other fish?