When your goldfish has a red tail, it is a symptom of a bacterial infection which causes tail rot. Check out pet stores and veterinary clinics for treatments to prevent further spread of the fungus which might be hard to cure. Visit http://en.wikipedia.org/wiki/Comet_(goldfish) for additional details.
Well he could possibly have an Infection called Ick short for something I can't say but it is contagious! My fish had it for three months and no medication has worked but what did work is raising the temperature of the water to 70 degrees if you have a heater this cures infection slowly but works it actually reduces slime and cleaning too!