Betta Fish: Will my New Betta boy Survive.

EDITED WITH MORE INFO:
I have brought one of infamous Betta cups. He was one of 4 half moon bettas in about enough water to just cover his body. The others were barely moving but this boy looked like he had some fight in him.
Brought him home and did the tank acclimation and then released him into more water then he would have had his life.
He went straight between the filter and stayed there for about 4 hours before moving out. My partner and I noticed him struggling to leave the surface and get to the bottom without floating back up to the top. The tank water is still but he drifts a bit. He is more mobile today and his ability to swim down is still abnormal but has shown improvement since yesterday.
I am hoping to give him a chance.
This was my first time seeing them sold like this near me and at my usual reptile/aquarium store.
Alarm bells on a SBD???
I Regret not testing his cup water to see what he was in.
I've lowered his water and offered plenty of spots for rest. Needless to say I'm sure this little guy has been through a rough bit so I know time. But what else can I do.
Yes I get weirdly attached to my pets quickly.
Hope

Tank size:20 litre
• Heater and filter? Yes
Tank temperature: 26.5°C
• Parameters in numbers and how you got them: API Master fresh water test kit.
GH 120°
KH 120°
Ph 7.6
AMMONIA 0
NITRATES 0
NITRITES 0
• How long have you had the tank?
Tank been set up to cycle for a week. Then I impulse brought the fish.
Did a seachem treatment prime and stability.
How long have you had your fish? 31 hours
:How often are water changes?
How much do you take out per change? No change yet any advice on when for this guy.
• Any tankmates? No
• What do you feed and how much: offered Betta bite pellets bug bite betta flake and a blood worm. Not wanting to eat
Decorations and plants in the tank: terracotta pot. Betta hammocks silk plants an anubias and java moss. Gravel substrate and a rock.