You need some plants including some floating ones, your tank is way to bright for a Betta and with no plants to hide in he is stressed.
The more plants the Betta.
One of my tanks.
PS
Just by looking at that photo I can see that the current created by your hang on back filter is too strong for a Betta.
The more plants the Betta.
One of my tanks.
PS
Just by looking at that photo I can see that the current created by your hang on back filter is too strong for a Betta.