Have you checked the ammonia levels in the tank? 3.4 liters is a bit less than one gallon of water, and ammonia can build up quite quickly in tanks that size. Also, is the water currently heated?
If you check the water quality and find nothing wrong, it could just be the frequent water changes that are stressing him out. Since your new tank is 15 liters, or approx. 4 gallons, you could potentially cycle it (I strongly recommend fishless cycling), which would allow you to cut back to only one partial water change per week.
Good luck with the upgrade :)