Betta, fin rot or tail biting?

I keep my betta in a 2.5 gallon tank with a filter and heater, one decoration that he can hide in, and a plant. His fins are progressively getting more torn up, and I don't know if it's fin rot or biting/tearing. I got some Bettafix to try to treat it but I don't know if there anything else I should do.

This is when I first got him a few months ago:




This is now:

 

Fin rot can come from poor water quality. Do you have your ammonia/nitrite/nitrate?

My friend's betta fish only tried to nip it's own tail due to stress from poor water quality and too small of a container, so I'm not sure why it would bite its own tail.

Hopefully someone who has more experience can help.

 

Fin rot can come from poor water quality. Do you have your ammonia/nitrite/nitrate?

My friend's betta fish only tried to nip it's own tail due to stress from poor water quality and too small of a container, so I'm not sure why it would bite its own tail.

Hopefully someone who has more experience can help.

I got my water tester the other night because my fish was acting weird (laying around and not moving much) and they said my nitrate was high, I did a water change and he's been acting okay since then, swimming around and everything. I've never actually seen him bite his tail or anything and he usually acts okay.

 

I got my water tester the other night because my fish was acting weird (laying around and not moving much) and they said my nitrate was high, I did a water change and he's been acting okay since then, swimming around and everything. I've never actually seen him bite his tail or anything and he usually acts okay.

It could be the decoration and water quality or just stress from only having 1 place to hide and having 1 plant

 

I don't believe bettas will very often intentionally injure themselves. Infection and fin rot are typically results of poor water quality and injuries from your decorations/other fish.

I am a total newbie when it comes to cycled aquariums, however I have successfully kept healthy bettas in uncycled tanks for long periods of time. I'm no expert but I'm quite familiar with these guys.

While he is recovering ESPECIALLY you want to do frequent water changes. His fins will never heal and grow back in dirty water. Doesn't always have to be 100% but maybe 50% every few days until he recovers. I always liked Stress Coat/ Slime Coat products.