This is why I highly suggest NOT using gravel/substrate when you bring back a new betta fish.
I kept all my girls in small tanks for the first couple weeks to make sure they're adjusting and eating properly.
The last one I brought home my little Sorbetto/Sorbetti didn't care there was no gravel. As soon as I released her into the mini kk tank. She immediately started to dance around the tank examining the fallen leaves on the bottom.
She even tried to eat one. You can see the leaf sticking out of her mouth in the 3rd picture.
Sorbetti's colors brightened up once I started to test her palate with different brands of pellets.
She's one of the smallest but healthiest bettas.