It's normally caused by poor water quality or an external protozoan infection. However, it can also be caused by fungus or bacteria causing the fin to clamp up.
Any chance of some pictures of the fish?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
Are any of the fish rubbing on objects in the tank?
Did you add anything new to this tank in the last 2 weeks?
What is the ammonia, nitrite, nitrate and pH of the tank water?
How often are you doing water changes and how much do you change?