I had two gold fish in a tank 1 was smaller than the other day by day I noticed that the smaller fish was losseing bits of its fins tail etc then I cleaned it out 1 day it had 1 fin left on it (poor thing) I don't no whether the other fish was eating it or it was bad! I took it out of the tank and put it in another tank, 2 days later my little fish had died do you no the cause of this??
I have one gold fish left and now that is starting to get red markings on its tail and body? Please help.

Sounds like a space or territory issue. The larger fish attacked and ate the fins of the smaller fish. Maybe a bigger tank or only 1 fish. How big is your tank?

Blood streaks in the fins is sign of stress in the water. Check all your water parameters(PH, Nitrites, nitrates and ammonia), this can stress the fish out.Your PH should be 7.0 an up for goldfish, 0(zero) on everything else. Water temp should be cool NO HEATER.

If the tank did not cycle to take care of the new fish, that's your problem. The bacteria cannot keep up with the fish waste. Feed every other day and not too much till the bacteria can catch up to the waste the fish is producing.

Do not change the water unless you show ammonia/nitrite etc. And always use a dechlorinator for tap water.