Goldfish and swordtails should -not- be kept together. Goldfish are temperate water fish whereas swordtails are tropical.
I rarely treat fin rot with medication. (Mainly because finrot is a rarity in our tanks.) But increased waterchanges and a small dosage of antibacterial medication would probably help minor cases. Antibacterials such as 'methylene blue'.
If you break down the cause and effect of the situation you'll see what I mean.
Swordtails nipping fin could cause irritation and possible infection -if- they also nipped the fish and caused actual injury. If they did not nip the fish itself, just its fins they probably ripped the fin a bit and nothing more. Ripped fins heal with time and good waterconditions as well as ridding one's self of the nipper. If however it was an actual infection it is most probable to be bacterial, as we all know bacteria is the basis of the aquarium's well-being. Fungal finrot can happen, but is much less common than some people think. (People sometimes suggest anti-fungal medication is why I say this.) If you are a person that'd rather medicate than try to treat with water conditions then there are plenty of medications out there, but as I said a bit of anti-bacterial and increased waterchanges should do the trick.
I would do 50% twice weekly waterchanges with methylene blue in the tank (or suppliment a low strength anti-bacterial medication of your choosing) and ensure not to overdose the tank.
The anti-bacterial medication just mainly cuts down on the chance of infection while the fish is recovering.
I will stop rambling now.
