Of course fish have small brains. They would look pretty silly if they had huge heads with large craniums.
Their brains only have a few duties, though. They need to stay alive and procreate. Their brains represent millions of years of imbedded information about feeding and survival. They do a great job at both and can be very selective when feeding on insects in their world.
http://caspar.bgsu.edu/~lobsterman/P...8KotVanHub.pdf