Must be the same reasons we don't run into all the walls. They must know there boundaries.

i dont really know. ask jeeves.

1) "In fish, the lateral line is a sense organ used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail. Sometimes parts of the lateral organ are modified into electroreceptors, which are organs used to detect electrical impulses. It is possible that vertebrates such as sharks use the lateral organs to detect magnetic fields as well. Most amphibian larvae and some adult amphibians also have a lateral organ. The Lateral line is related to the Ampullae of Lorenzini. The receptors in the lateral line are neuromasts, each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula, typically 1/10 to 1/5 mm long. The hair cells and cupolas of the neuromasts are usually at the bottom of a visible pit or groove in the fish. The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, indicating that the lateral line and the inner ear share a common origin. Teleosts and elasmobranchs usually have lateral-line canals, in which the neuromasts are not directly exposed to the environment, but communicate with it via canal pores. Additional neuromasts may appear individually at various locations on the body surface. The development of the lateral-line system depends on the fish's mode of life. For instance, fish that are active swimming types tend to have more neuromasts in canals than they have on their surface, and the line will be farther away from the pectoral fins, which probably reduces the amount of "noise" that is generated by fin motion. The lateral-line system helps the fish to avoid collisions, to orient itself in relation to water currents, and to locate prey. For instance, blind cavefish have rows of neuromasts on their heads, which appear to be used to precisely locate food without the use of sight; killifish are able to use their lateral line organ to sense the ripples made by insects struggling on the water's surface. Experiments with pollock have shown that the lateral line is also a key enabler for schooling behavior. Some crustaceans and cephalopods have similar organs." Source and further information: http://en.wikipedia.org/wiki/Lateral_line 2) "The fish aren't using their eyes to "see" the glass, but rather a special pressure-sensing system called the lateral line. This system is made up of sensory units called neuromasts, consisting of cells on the body surface that have a projecting hair encased in a gelatinous cap. When pressure waves in the water move the gelatinous caps and bend the hairs, the firing rate of nerve impulses sent to the brain by the neuromasts changes, enabling the fish to detect the waves. In fish these neuromasts are mainly found in a groove running along the side of the body, hence the term lateral line. Branches of the lateral line system extend onto the head as well. Besides fish, the system is found in some aquatic amphibians. Fish set up pressure waves as they move through the water, and are able to detect the reflection and distortion of these waves from objects and thereby avoid them. They are also able to detect the pressure waves of other fish. The lateral line system can also pick up low frequency sound (a kind of pressure wave) at 100 Hz or less. Certain species of blind cave fish are able to navigate perfectly well in their lightless environment by using their lateral line system alone." Source and further information: http://www.straightdope.com/mailbag/mfishbowlcrash.html

they prlly want free

The same reason we don't run into walls. Spacial awareness occurs even for those blind at birth. (see iwnit's answer)