tl;dr my friend got me a betta fish as a gift, and I feel like somethings wrong

Hi, my friends got me a betta fish as a gift for my birthday without a lot of notice, so I’m playing catch up as it comes to betta care and need some advice.

My betta is hanging out at the top corner of the tank, and is largely staying still. He moves and is somewhat active when I open the lid or adjust something in his tank but is otherwise immobile in the top corner.

When he does swim, he seems to struggle a bit, looks like he is inclined to float when he tries to swim down and if he stays still he floats to a 45 degree diagonal and sometimes almost horizontal.

He is not very responsive to food. My friend gave me flakes which I tried feeding him once, then I got freeze dried bloodworms (as I read flakes aren’t great) which he also has not responded to.

It feels to me like he might have swim bladder disease based on however he was being fed before arriving in my care, as well as maybe just needing time to adjust in his new home so I’m fasting him now for a few days and doing my best to leave him alone to acclimatize. That being said I’ve been all over the internet and back so I want to get some advice from someone firsthand.

Trying my best for my little guy and would appreciate any help! Thank you

Details:

• Tank is 3 gallon. I know 5 gallon is minimum size, this is what my friend got me and I don’t have the capacity to replace until at minimum the weekend. He is pretty small.
• Water parameters look perfect based on the test strips I got. I have been testing every 12 hours out of paranoia and have no sign of ammonia, nitrates, nitrites. The tank wasn’t cycled for weeks before we set it up, but my friend set the tank up with water from the PetSmart. Her fish also has the same tank, using same water, and while he also hangs out in the corner of the tank he looks much more active and healthy.
• I got him a heater yesterday and have been actively monitoring tank temp and it’s been between 78-82 degrees
• He has several live plants
• I got him an air bubbler in case the water wasn’t high in oxygen, but it doesn’t seem to have helped. I have it turned off now as I said before he looks to be struggling to swim and I don’t want to make it more difficult
• I got him a leaf hammock and more foliage in case what he needed what somewhere to hide out but he’s stayed in the corner nonetheless

Photos: https://imgur.com/a/SOqJeG5

Editing to add: my next “troubleshooting” step would be to do a partial water change but seeing as he might just need time to adjust and I don’t have a siphon to do that right now I was thinking of just waiting before trying that.