I'm pretty sure that it has something to do with the water. My VT's fins curl and mess up when he's at college with me, and start to get better when I take him home over long breaks.
Have you received any notices from your water company lately? My school is treating it's water with extra chemicals, has found bad bacteria in the water, ect., ect. And now he has bad fins.
That might not be the problem, but you can always call your water company and check if anything is going on. At least if this isn't the problem, you can eliminate it as a possible source.