I used to keep one of my Bettas in a 30g. I kept the water line about 3-4 inches below the top of the tank to make it easier on him
Its not too big but it might be too big for you to keep a betta in there on his own
Some things you could do is put a few other fish
Non aggressive ones, I may add. They may nip at a bettas fins
Also, avoid guppies, because bettas may mistake them for others of their type and attack them
You could even try with some ghost shrimp and/or snails. To make it more pleasing to the eye, and to keep the fish in a more natural habitat, put some live plants in.
I typed all this with a half asleep kitten attacking my finger. I really need to clip her nails again.
You could even put 3 or more female bettas in, because with that they may have enough space to be ok with each other.
Any Betta tank larger than a ten needs to be heavily planted to allow easy access to the top, so that's a must. Are you thinking about a single Betta or a sorority? For a sorority you'll need 7 females at least. Do not keep a male with a female or another male. As well as guppies, don't add anything flashy or gouramis.
Agreed with Platylover . Beta's need easy access to the surface so tall large leafed plants like swords or crypts or very mature java ferns are good choices for them to be able to rest on near the surface.
If you are going to keep a betta in this with other fish, I'd suggest something small and very fast but at the same time very peaceful. Harlequin rasboras or cardinal tetras would probably be ok as I don't believe they are nippy at all and are fast enough to evade the beta.
Thanks everyone sorta swaying me away from a betta now
Multiple species of tetras
Guppies (I'd recommend male only)
Maybe an angel, but not sure
Those are a few you can put in there. Not super sure which ones are compatible though.
For just 1 betta a 5 or 10 gallon would most likely be a easier/better idea. They still would like the floating plants to lay at the top of the tank.
I have a few in 5 gallon and one in a 40 gallon breeder.
He lives with black neon tetra, threadfin rainbow fish, blue ram and thick lipped gourami. He seems really happy in his 40g. I have a lot of pothos (and water sprite) floating from middle to the top and out of the tank. He really like laying on the leaves that will keep him just under the water. He always comes to say hI to everyone even my cats that like to drink out of the tank.
I have a canister filter I made a spray bar for that runs the back of the tank and splits the water up so the flow doesn't push him around.