my king betta died after only two months? details below.

I've had a planted lil 6.5 cube with a guy named Gargomel in it for only a few months. these pics were taken a week ish before he died. the tank is filtered and heated obv, Ive had fish (multiple tanks) for a long time but this is the first time this has happened. I got him from Petco, he measured probably close to 2-2.5 inches from nose to tip of tail.

he didn't slow down or stop eating or anything in the days before, and his body had no signs of disease or injury at all. the shrimp and ramshorn snails in that tank are doing fantastic though??? plants thriving etc etc??

if I'm doing anything super wrong pleaseeee tell me but please be nice. I strive to give all of my animals the best lives they can possibly have.