I've had my male betta fish, Nacho, for almost 3 years now. A few weeks ago I noticed his odd behavior, not swimming, lethargic, buoyancy seems a bit off... so I ruled out that he must have swim bladder. (probably because I would feed him his favorite blood worms too much
I've stopped feeding him pellets and worms. So now I lowered the water level of the tank, I've been fasting him for about 4 days, then feeding him a frozen peeled pea. I repeated that process about 3 or 4 times. (the temperature of his tank usually reads 78-82)
Lately it seems like he's just getting worse!

He's not even bothering to swim or look at his food anymore. He just lays almost on his side or stays in the corner of his tank.
It's really depressing and bothering me

He use to be such a happy betta, making his cute bubble nests, and greeting me every time I entered the room

I'd really really really appreciate some advice. Thanks.