How long have you had your fish? He could use a nice floating plant in there to help him feel more comfortable.
He looks healthy.

 

We have had him since Christmas. I just don’t understand why his dorsal doesn’t open/flare. Thx for advice on plant!

 

Hi, how long has the tank been set up before you added Betta? How big is tank? How often you do water changes? Can you give parameters?
Ammonia
Nitrite
Nitrate
Ph.
Numbers please, not just fine.

 

We had the tank set up for at least 24 hours before adding betta. It’s a 2.5 gallon tank. Once a week water changes. Ammonia-0 Nitrite-0 Nitrate-0 Ph-7.2

 

That small of a tank will need water changes daily. Its not cycled and very hard to cycle that small. Add some fast-growing plant like hornwort to help. Get a 5- or 10-gal tank with a filter, set it up and cycle it before putting betta in it.

 

This tank does have a filter and heater. Does that make any difference?

 

it does some but a betta needs temps 78 - 80, and the tank isn't cycled and will need water changes daily and live plants to make him happy.