Moved....
Got any pics? Fin rot is bacterial(not uncommon) and it also could be a symptom of a under lying condition, such as HLLE.
It could also be something a simple as picking/fighting and not fin rot
It is normally from less than desirable water conditions... considering you just picked him up... it could be from where you got him, or perhaps he was just recently shipped, and got it in the shipping container(most likely).
List your water Perimeters... Specifically ammonia,trites,trates,phates....
Keep up on water changes, supplement Garlic(helps the immune), If it gets worse, you can QT him and treat with antibiotic(Maracyn)... but as bad as it sounds you need to weigh out if he is worth that treatment or not....
The other fish can get it, as it is bacterial, however their immune systems, provided they are not supressed, should handle it just fine, supplement garlic to assist with theirs, plus his immune.
It is not something that you would need to strip your entire tank for or start over for...
If you decide to dose antibiotics... DO NOT DO IT IN YOUR DT.... QT THE FISH... it will wreak havoc on your biofilters.