2 small red veins and off coloured rear fin

my two common goldfish have all of a sudden started to chase(harras) my redcap fantail so i have now taken it out of the tank for closer inspection and found that it has a slightly off colour rear fin with 2 very small red veins in it and the fish also seemed a little tyred and distressed when it was in the tank with the others.their is no dammage to any of the fins and the fish is eating ok swimming ok etc ive spent all night looking on net to try and find out what was the problem and came up with 1 million possabilitys and no real answer i think it could be stress as its new as are my moorfish and 3white cloud minows to my new tank,but all the other fish were bought together and put in at the same time apart from the two original goldfish(common) that i had previouse in a goldfish bowl then intraduced them to new tank and other new fish ie white clouds and moor and redcap,untill the two common started to harras redcap. im not sure what tratment if any to give the fish so could anyone help..
once again symptoms are (slight discolouration of rear fin and 2 thin red viens in rear fin only in rear fin) it also seemed harrased (mind you i would if i was being chased all over for hours on end) I also wondered if the two common goldfish were trying to mate with my recap is this possible...........................please help too many possibilitys totaly confused.new dad daz