So this is pretty weird for me. I got a new fish two weeks ago to go into my 10g, since the previous betta passed away. Said fish didn't pass inside of the tank-- he got a bit sick so I QT'd him, and he wound up passing in his QT cup after two weeks. I went to petco to shop for another male, and brought a cute halfmoon who seemed perky and alert in his cup. He was a bit stressed on the ride home, I floated him in the 10g like normal, acclimated him to the water over 2 hours and uncupped him.
Didn't see him for pretty much a week. I've had scared bettas before though so I figured he'd warm up. I finally got worried, and found him hiding in the back of the tank. I decided to pull out my 2.5 gal and set that up with gravel, a little cave, a floating betta log, and a plant-- ie tons of cover, and put that in a quiet spot where I could see it without getting close. He ate in that tank, but just hung in the corner all week, clamped and stressed and getting pale every day when I fed him.
Today I put a little AQ salt in a cup and cupped him, because after my feeding he was so scared he just laid at the bottom of the tank gasping. He seems much more perky now-- he's colored back up and is watching me, though he's still clamped a bit. I'm not sure if he's sick or if he's just stressed-- anyone every had a fish that was actually scared of a big space? :/