I saw a jaggedness of the tail fins or raggedness on the very ends of the tail fin. Can you see it? That most likely is fin rot.
Fin rot sometimes isn't noticeable until it's almost too late to treat the fish. Clamped fins most likely means that the fish is stressed or in shock. Almost always a fungal infection will follow a bacterial infection, where there has been damage to the fins or skin. Not always but usually.
If you get more fish from the petshop, I would look them over careful before buying and when you get them home, treat them with a preventative such as melafix. I take my handy dandy magnifying glass with me to the pet shop and examine the fish AFTER it is placed in the plastic bag.
If it has sores, torn fins, or something that I missed in the tank, I reject it and choose another one.
Fish that have a disease or are carrying one, can get stressed from even the slightest amount of ammonia or nitrite in the water. It is always best to make sure your tank water readings are at zero for these two tests before placing a fish in the tank.
And one more thing, just because a fish looks perfectly healthy doesn't mean that 2 weeks from bringing it home, it doesn't get sick. Some diseases have a 2 week or more incubation period and don't show up for a couple of weeks after you get them home. That is why quarantine is really a good idea before adding any new fish to your tank. I usually quarantine a new fish for 3 weeks but some experts like to quarantine for a month or so.