I have a 2 gallon tank with no filter and he has rocks for the bottom,and he has a place to hide,he seems like a happy fish,I've only seen him 'puff out' his fins and get angry twice,I've had him for about 2 months..Can I introduce him to other fish? if so,what kinds..and will I have to make any changes? Also,how would I go about actually introducing him to others?