Do you see any black around the edges of his fins? Black edges indicates fin rot. Is there anything in the tank he may have ripped them on? It's also possible that he's tail biting.

 

No, I don't see any black around the edges. I have a silk plant and a little cave rock for him to hide in. I had them both when I kept him in the fishbowl, and I just transferred them over when I got him a tank a few days ago... Is it possible that it is just now ripping his fins so bad?

 

So the edges of his fins look normal and he was just put in a new tank a few days ago? It's possible that could have triggered it, I think. Can you describe "tattered" a little more? If he is tail biting you may be able to see a kind of half moon mouth shape impression from where he's been biting.

 

It kind of looks like this picture, but my betta is blue and black.

 

Also, his tail is a little shorter.

 

That looks like betta mouth shapes to me.

 

How would I stop him from biting his tail?

 

Tail biting can be caused by a lot of things. Getting it to stop depends on figuring out why he started doing it. Since you just put him in a new tank, it's possible that's why he started.