Hi Kristina. Since this little guy is already in quarantine as a newly purchased fish and is currently undergoing treatment/observation for either constipation or swim bladder concerns, I'd suggest you update your existing thread with new information like this to clarify the full situation for others who might respond here (or there) and so we may offer more useful advice until he's fully recovered. For that reason, I'm going to merge this new thread into your existing one. (Will note this within the thread, as well, so it's clear for others reading from one to the other.)
Fin rot is very common as a secondary complication along with other problems because it's often caused by highly stressful environmental conditions. Goldfish (like bettas) often react to stress this way. Poor water quality is another likely cause, so be sure you keep up with your water changes and monitor the parameters (ammonia, nitrites, nitrates) very closely. The main thing to watch out for right now in regards to his finnage would be (1) reddish areas along the edges or (2) very rapid deterioriation.... which may indicate a bacterial infection (as with Aeromonas, Pseudomonas, or Flavobacterium).
In that case, topical treatment using peroxide, followed by some neosporin may be advised. Keep in mind, however, that most people over-treat fin problems with antibiotics before they correct water quality issues or eliminate other stressful factors. Since he's already in quarantine now and we know the stress he's dealing with to begin with, that already gives us a likely explanation for what you've now discovered.