Cecil Adam's explains why fish don't crash into the sides of their tanks despite the lack of binocular vision.
I'm not sure if my goldfish lacks this sense or just enjoys pain. He slams into the sides and lid of his aquarium on a daily basis. I have a large skull in the bottom of the tank and within a day or two after I clean the tank, he has it knocked over.
I never intended to have a goldfish but my step-daughter brought home a bucketful of tadpoles a few years ago. As they metamorphed into frogs, I noticed a tiny fish swimming with them. My wife, a biology major, informed me that it couldn't be a goldfish even though the water came from a pond full of koi. Within a few weeks, it grew into an obvious goldfish and ate the remaining tadpoles. He's lasted for several years, much longer than the fish I bought or won in festival contests.
According to the lists of facts that people feel compelled to e-mail around (which usually include blatant lies like "duck's quacks don't echo and no one knows why"), goldfish can only remember things for three seconds. This fish remembers that when his light is turned on that it's feeding time and surfaces for his breakfast. He's either very smart, a mutant, or you can't believe the goofy crap that people e-mail you.