Does anyone know what this red line is on the tail fin of this guppy?

http://www.youtube.com/watch?v=hac1_evAkdE

I went away for Thanksgiving break and left the tank unfed for four days. When I came back, he looked like this. The tank has been set up for a few months now with no recent change in its water parameters. When I came home I measured the concentration of things. The nitrate was 20 ppm, chlorine 0 ppm, and pH 6.5 ish. The hardness is close to 0 for DH and KH. I don't add salt. The fish are normally fed flake food three times a day. It's a 10 gallon tank with just guppies in it, and he's the only male with about 10 females.

I have no idea what's wrong with his tail. I did a water change to try to help him, but it would be nice to know what's going on.

Strange. Has he perked up since the water change? You could try a general antifungal/anti bacterial medication to see if it helps, but i have never seen anything quite like that.

He looks great! Glad he recovered!