Fin Rot?

I got four juvenile corydoras julii less then two weeks ago. I broke my own rule and did not buy them from my favorite, excellent fish store, but from another store where I occasionally buy supplies. They are far better than a Petco, but they do not have ideal conditions. But they had four healthy-looking, pretty young corys and I caved and bought them. I noticed that their dorsal fins looked a little ragged, but not enough to alarm me. Now they've gotten worse and I am worried. All four corys have a chunk missing out of their dorsal fin where the black spot is, all in the exact same place, all nearly the exact same size. I actually looked them up to see if this was some new fancy strain of corys with weird fins, but no. The holes are very gradually getting bigger. One fish has an exposed ray. All of their other fins are in pristine condition, and they are very active and eating lots of food. Except for their fins I would say the are in perfect health. None of my other fish are sick. One of my corys does have some missing barbels, but she might have come to me that way and they are growing back. Is this fin rot? How do I treat this? I'm attaching a picture, but it's not a very good one. All I had was my phone, and the cory didn't want to sit still and pose. Also, don't judge him by the color. I edited the color to try to provide enough contrast to make his dorsal visible.