yes if the mirror is there too long the fish see's it as a invinsible fish that dosnt back down and he will feel dominated. the mirror is to stimulate dominance in your fish.
by puting a mirror next to the tank your fish thinks it is another male fish, and is
a threat to his territory and will try to fight him back. when you remove the mirror
your fish will feel he pushed the fish back and feel dominate. so make sure you dont
put the mirror there too long.