Betta with a lump on his side!

[Salty&Onion]

I want to just throw this in, he's also bloated, his pectoral fin looks to be ripped a bit and his tail looks like tail biting or fin rot. He's also lethargic a bit and is hiding a lot.

 

[PheonixKingZ]

Do a large water change. (60%)

Are any of his other fins looking ragged? It could be because you just got him. It’s a new tank and a new world for this guy. Relax a bit...

 

[Salty&Onion]

Before putting him in the tank, actually before going to my lfs to get him I did huge water change and vacuumed the substrate and planted the anacharis.
Later on, today, I will put in a heater for him. What temperature (in Celsius) should I set for him?

 

[FishFinatic77]

25- 27 degrees Celsius should be fine. And yes, it could just be a muscle bulge. My betta has one too.

 

[Salty&Onion]

25- 27 degrees Celsius should be fine. And yes, it could just be a muscle bulge. My betta has one too.

I'm happier that it might be just a muscle bulge.

Update:
He swims around more!