i just bought myself some gold fish about two weeks ago, i had bought 6: one died two days later and the others were pretty active.
originally one of the goldfish had half of a fin on his tail...then the whole thing disapeared and the end of his tail has a whitish color. the rest of his fins are close to his body and seemes to have some blood like vessels appearing next to the dorsal fin.... hes active, still eats with the rest of them at feeding time...but i put him in his own tank now...would anyone know whats the matter? why did his tail disapear? are the others eating him?

Well first of all, what size tank were these 6 goldfish placed in? Goldfish need 20+ gallons a piece, despite what some tend to believe. Many times you'll even hear "10 gallons per goldfish" but even thats inaccurate because if you ever seen a fully grown goldfish, he wouldn't be able to turn around in such a small aquarium. <_<

Anyway, we will probably need to know your water parameters (ammonia, nitrite, nitrates, pH) before we can make a "for sure" diagnosis. Many times the bleeding of the tail means excess ammonia or very high nitrates. This is NOT good because this means there is a serious problem. It could also be a bacterial infection (internal) if nothing else. Maracyn 2 works good for bacterial infections but first we need more info... ;)

Not having a lot of the information that the previous poster mentioned, I would guess that the injuries are physical if the other fish don't have them. Probably, the tail got torn. Then, the other fish see if like a wiggling worm and can't help but bite chunks of it off. On top of that, bacteria and fungus like to take hold in the injured fins. As long as one fish is unable to swim as fast and is ill or fancier in nature, the other fish will pick on it. I suggest keeping him separate at least for now. I would treat with aquarium salt (a tablespoon per few gallons), MelaFix by Aquarium Pharmaceuticals, and Marcyn by Mardel (antibiotics). If tufts of white fuzz appear, add an antifungal medication as well. The biggest thing though is to get him away from the other fish who will continue to eat his fins. The blood streaks in the fins are septicemia or an internal bacterial infection that may be due to stress (having your fins ripped off is stressful!) or water quality (hence, do check your pH, ammonia, and nitrite levels at the very least). Good luck, and I hope he gets better!