Betta has a lump or blister on his fin

Hello!
My new betta fish, Jasper, who I got on the 6th of January, has some sort of odd lump thing on his ventral fin. I would try to take a photo if I could, but it's quite small and he is super active, so it wouldn't show up on the camera. apologies! I cannot tell if it has grown at all as it is hard to get a good look, but I can see there is something. My past fish, Zuli, had a growth on his dorsal fin which got bigger and eventually killed him (at least I think it did, he went down hill and passed away quite young, not long after). I may be over reacting, but it really worries me that I might have to go through the same thing again with Jasper. The lump on his ventral fin is quite small, and the same color as his fin. The other thing about it is I have begun to see some fraying on the same fin! I don't recall this being there before, although I may have missed it. I was thinking it could be the piece of dragon stone in my tank ripping up and damaging his fins. I sanded it down before adding it to the tank to prevent this, but I may have missed some spots. However, all of his other fins are fine and undamaged which seems a bit odd if that were the case... Possible though. My other thought was maybe he is getting damaged fins from stress..? He is always swimming back and forth in the front of the tank, as if he is trying to get out. My tank is a 5.5 gallon and decent to heavily planted.

Another thing I noticed about his behavior is that ever since I got him he has been very picky with food. He usually eats brine shrimp, but with any other food, he will put it in his mouth and then spit it out.
I will post my water parameters soon. Thanks in advance!

 

Image pls and thank you!
The swimming back and forth in front of the tank is called glasssurfing it’s normally a sign of stress or so thing that bugging it

 

As I mentioned before, I am unable to get a photo. Sorry. And yup, I know about glass surfing, I just am not sure what's causing his stress. Thanks for your response!

 

Ok, didn’t see part of can’t get picture. I wish I could see bc it would most definetly help me with a diagnoses but it’s ok.
What colour of lump is it?

 

The lump is the same color as his fins... Also, now he really isn't eating anything :/
It might just be because he is stressed since I just did a large water change, but he wasn't eating yesterday either. He will put a brine shrimp in his mouth, spit it out, and then will ignore any others I offer him. For the first few days after introducing him into my tank, he was like this -- very overly stressed and active and not eating anything. He gradually got better and more comfortable with everything, but then it all flopped again; not sure why.

 

does it look like a tumar or maybe like a absess? do some reasearch on what both look like then inform me what it looks for like. for now you can add some aquarium salt to the tank, or stress coat. you can try and dip his brine shrimp in garlic sauce.