Unfortunately, he seems to have gotten worse. Now most of his fins have frayed edges. I almost wonder if his fins are more fragile than other bettas because they're transparent? I did a 10% water change for his 10 gallon tank (which I plan on doing weekly), and the new water has a dechlorinator with Indian almond leaf extract. However, when siphoning the bottom I ended up mixing the smooth rocks with the gravel so they don't cover the gravel as thoroughly.
Should I remove the gravel completely and use smooth stones/glass? Or would that destroy the bacteria? I also have a piece of driftwood, but he never goes near it so don't think that's part of the problem. Thankfully his fins don't seem to be bothering him, but I'm worried he might be at risk of infection.
Sorry to bombard you guys with so many questions! I wish I could find a good article on it, but they all seem to be about fins that have parts completely torn off instead of fraying.
Here's a picture of how he looks:
and here's what the gravel looks like, mixed with the smooth stones and glass: