Actually, it would be ammonia poisoning.
2L = 0.264172 gallons.
That is not even half a gallon.
With a "under half a gallon" you will need to do an every day water change, 100%, to ensure the ammonia stays down. Though he has been with you for a few months now, now he is unable to fight off the dangerous amounts of ammonia building up...
I would also suggest maybe making it easier on yourself, and find at least a 1 gallon bowl (or 1.5 even better :)) which then you can also find yourself a heater that will get the temperature to 78 F degrees. Since you are in Toronto, we both know it can get mighty cold outside
Unless you heat your house to 32 degrees celsius, the fish won't be warm enough... Plus not to mention the dangerous temperature fluctuations!
Have you been adding tap water conditioner as well? How much is he fed, and what is he fed?