Betta being lethargic, not eating, and tail nipping!

Yesterday I got home and checked up on the betta I rescued from petsmart a month or two ago, he was laying straight up on his amazon sword plant, so I stuck my feeding tweezers in there to check and see if he was okay. He moved and went to the bottom, he wasn't moving very much. So I did a 90% water change after I saw this happening. As I was doing the water change, I noticed there were small holes in the back of his tail!
He had a plastic pot to hide in so I took that out just in case he was tearing his fins on it. Later on, I also noticed his tail was almost halway gone! ( he's a delta tail so his tail is pretty big) I'll try to include some pictures, but what the heck should I do?

I'm gonna try to give him a brine shrimp and see if he'll take it

 

What are your water parameter? I’m no expert on bettas, but when my guy had some fin issues indian almond leaves and small daily water changes worked wonders.

 

Yesterday I got home and checked up on the betta I rescued from petsmart a month or two ago, he was laying straight up on his amazon sword plant, so I stuck my feeding tweezers in there to check and see if he was okay. He moved and went to the bottom, he wasn't moving very much. So I did a 90% water change after I saw this happening. As I was doing the water change, I noticed there were small holes in the back of his tail!
He had a plastic pot to hide in so I took that out just in case he was tearing his fins on it. Later on, I also noticed his tail was almost halway gone! ( he's a delta tail so his tail is pretty big) I'll try to include some pictures, but what the heck should I do?

Tank size? Water parameters? It's possible he is fin nipping. Bettas often do this when they are frustrated, either by how their fins slow them down, or out of boredom.
I would do 25% water changes very other day and add a couple Indian almond leaves.

 

Thank you! I'm gonna get test strips later! So I'll fill it out then!

Tank size? Water parameters? It's possible he is fin nipping. Bettas often do this when they are frustrated, either by how their fins slow them down, or out of boredom.
I would do 25% water changes very other day and add a couple Indian almond leaves.

He's currently in a 10 gallon divided tank with my other Male betta.
Where are good places to get Indian almond leaves?

What are your water parameter? I’m no expert on bettas, but when my guy had some fin issues indian almond leaves and small daily water changes worked wonders.

I'm gonna get some test strips later so I'll tell you when I know
: D

 

just a side note for the future it's really not good to do such large WCs because it can cause dramatic swings in parameters and stress the fish out.

If you're able to I would get a liquid test kit, they're far more accurate than test strips and I know a lot of strips don't test for ammonia either which is super important to know.

 

Thank you all for your help, but my little buddy passed away last night. I'm glad I was able to help him live a little longer than he would have when he had swim bladder disease in that tiny, cold, cup at petsmart.