Good. I don't think though its the matter of size because both tanks are not really big. But i think keep him there for now. Check for the water conditioner tomorrow, check your water, also you can still let the water sit for 24-48 hrs for the next water change .
You can just rinse the 2 gall i guess with warm water until we figure out what wrong and you can put him there again.
Not sure what water conditioners you have in your stores since you in Serbia. But just look for the water conditioner that Removes Chlorine, Chloramine and Ammonia,Detoxifies Nitrite & Nitrate. Prime by Seachem is the best water conditioner. If you confused with water conditioner google it or let the forum know people will help you .
I hope he will feel better , let us know how he doing.