Betta Fish: What is wrong with uncle bill?

Hi, I got uncle bill here around two months ago from someone who could no longer take care of him and I noticed that his fins are clumped/ripped? He was like this when I got him and other than this he eats well and nothing is out of the ordinary. I switched over to a sponge filter for reference, there is no one else in the 5 gal tank, and I do filter changes every week (~25%) and every three weeks at ~75%. Any advice would be greatly appreciated