At night, the sole illumination is the soft glow of moonlight, and that only happens during a portion of each month. Even during the full moon water turbidity often limits moonbeams to the upper portions of the water column. The problem for fish is that they rely heavily on their sight to feed and this reliance increases dramatically when the water muddies. Since this situation may occur for days at a time, it's obvious why evolution favored species that developed keen hearing. A fish starves if deprived of nourishment for long enough.
Fish hear in three ways.
First, they have ears called otoliths. They are located in much the same place as human ears, one on each side of the head. But, they differ from humans in that they do not have openings to the body's surface. Instead, sound waves travel through the skin, flesh and bone of the fish's head to the otoliths. Hearing done in this manner is directional just like ours.