Why does my bettas tail look like this?

His tail wasn't choppy like this when I got him, I've had him for almost 2 years. He switched from my mom's tank to my new tank on Christmas last year, I used miracle grow soil at the bottom for my live plants. His tail begun to get choppy before I added him to his new tank, and it's only gotten worse. Water quality is about perfect, I do nitrate and ammonia tests every couple months.

Sorry about the low quality photos, I don't have any better cameras.