I have a 10 litre bowl that I can swap them into?
I have just done a full water change and the betta looks better already, so I am guessing you were correct with ammonia overload.
I have had my male betta for 4 months in a small 1 gallon (3.5 litre) bowl. The bowl had a plant and the temperature is regulated at 26 degrees Celsius at all times. My betta has been perfect, making bubble nests and everything all the time.
The guy at the aquarium suggested that I introduce a...