Betta Fin Rot, Or Damage?

Hello all, new here and I hope I'm posting in the right place, and not breaking any guidelines.
So I've had my betta for about 3 months now, and just recently I've been noticing some fin problems. He's in a 5 gallon tank with a filter and heater. I treat the water with API stress coat after evevy change, roughly every 2 weeks around a 70% change of water.
He has a little plastic house decoration, and had a plastic palm tree type, but I removed it in case that's what the issue is.
Basically I've just noticed his fins having issues, and I think I keep it pretty clean, but I'm just wondering what I should do next. He's still lively, and eats all his food, just getting a little short fin.
Thanks

 

How strong is the filter intake?

Also welcome to fish lore

 

Yes welcome. Do you test your water? If so what are your parameters?

 

I don’t think it’s fin rot because non of the other fins are infected couldn’t be wrong though

 

That’s the worst part of not being able to see! But fin/tail rot can affect one without the other for some time. But first we need to make sure it’s not the water quality. @Hunter1 can you see this bettas tail?

 

I'm not sure, it's the tetra whisper 10i which is supposed to be from 3-10 gallons, that is one of my concerns so ivI' only been running it whenever I'm home so I can keep an eye on it.

I haven't done any type of testing, just regular water changes really.

Would something like an API 5-in-1 test strip kit be sufficient?

 

Since the damage is mainly on the Caudal fin, Id watch him to make sure he isn't biting his own tail. Here's a picture of tail biting.

 

API liquid master test kit. Strips are not reliable. Have you compared your fish to the tail biting pic posted?

 

I was wondering that also. When I first got him he was pretty calm, he even made some bubble nests for a few weeks. Then it seemed like he started attacking his reflection more, and being a little aggressive, I haven't notice nipping, but I'm not home all too much.

 

Some people who had nipping problems notice that their fish only do it in the dark, so that could be why you never see.