So at first my fish were swimming fine and now, ever since I added a new goldfish it has started nibbling the other goldfish that fish is a fully grown comet. Slowly the comet goldfish has been getting sickly and started to show signs of tail rot also infecting all the other fish I have (which they are 1 koi, 1 pleco, comet goldfish). So as my comet goldfish is sickly, the new goldfish keeps nibbling. Since all of my fish (except for the new one) have tail rot, should I should quarantine the new one until the others get better? Should I also add some medicine for tail rot?

P:S: I did a 50% water change and changed the filter as well, but it doesn't look like they are getting any better. I also had the water tested by the Pet Store and they stated the nitrates were to high (although I fed them in the morning). The Pet store stated that I don't need to add medicine,
but they have been sick for almost 2 days now.
I don't know what to do at this point, please help!!!

You have a koi, goldfish and a pleco altogether??? This doesn't sound good at all. When you cleaned the filter did you 100% clean it as in in tap water or completely change everything? If so you shouldn't have done this. Only clean half the filter material at once in some tank water which will keep all the good bacteria in there. Could you also get your water tested again and ask the pet shop for the actual numbers of the test? This will give you a better idea of what to do and we can help you a bit more maybe

I would recommend to separate all three fish as they all have different needs. For now keep up the water changes, testing the water and treating with the medicine.