Betta Fish: Newest betta has an extended gill; how can I treat this if it doesn’t go away?

Hello! A good handful of people have seen already my newest betta who has yet to be named.

I got home from work today and went about my usual routine of feeding each of my bettas and took some time to check on the new guy and see how he’s settling into his new tank.

The tank was just set up very last minute, but I’ve never had an issue with any of my bettas having extended gills after being put in. Please note I’ve owned bettas for 10 years and for the last 3-4 years have set up all the tanks the same way when doing cleanings and bringing home new ones.

His tank is very clean as it’s a fresh out of the box tank, along with the decor, the only reused thing is the rocks (they came from an older tank I no longer use) which were thoroughly cleaned with hot water before being put in. The tank cycled for 3 hours while he acclimated before being put in. He’s currently in a 3.5g tank (I’m aware bettas need at least 5-10g, but this was what I could afford last minute as I really wasn’t planning to bring home a new betta last night..)

Looking at the photos I have from while he was acclimating, the gill was extended then too, I’ll put that photo in the comments!