Looks like a 3 gallon tank,
Mine all love Bubble walls or if not enough room a small stone, But you will need to adjust it to his liking.
I ended up discarding most of the Cave thingy in the picture, None of my fish seemed to like it to much,,
I found alot of my betta's fell in love with the small castles, At first glance you'd think they'd never fit in them, But its the perfect size for 1 Betta,
Mine likes to slip in thru the back hole and hide while peeking out the front, When they see you looking they make a grand entrance thru the front door in a big flurry, I think they figure there MOVIE STARS!
The castles come in a variety of types,