Start of clamped fins?

Hi!
My new betta (double tail half moon) has been living in his 3.5gal for 4 days now after buying him from PetSmart. Hes very active but today i noticed the top of his double tail seems a bit clamped. The tank has a heater, temp is around 77-78F, filter and testing for ammonia this morning resulted in 0ppm. (The tank has been cycled for a while now) Is this slight clamping a cause for concern? Is he stressed? The second photo is the day i bought him for comparison. Any help or input would be appreciated. . Thanks

Its gotten a bit worse since this morning. I just did a 25% water change.

Do a 75% water change and gravel clean the substrate every day for 2 weeks. See if it helps.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Monitor the ammonia, nitrite, nitrate and pH of the tank water.