If your fish seem to flock to the other side of the aquarium when you and your friends press your faces against the aquarium glass, it's because they can see you. Most fish have some sort of eyesight, although not all fish see the same way. They often use other senses such as hearing, smell and touch to supplement their vision.
Basic Fish Eyesight
Most fish have eyes that work similar to those of land vertebrates. Their eyes have the same basic anatomy, including a lens, pupil, iris, cornea and cones. Fish with eyes on each side of their heads can see to the left and right at the same time; their heads don't turn on necks like many land animals, so this allows them to see in front of them and to each side all at once. Scientists haven't researched fish eyesight extensively, but they agree that most fish possess the ability to see.
A fish's ability to see color seems to correlate with where he lives. Fish who inhabit shallow, in-shore areas tend to have enough cones in their eyes to see several colors, such as red, blue and green. Sunlight penetrates the water in shallow areas, which helps colors shine through. These areas often house coral reefs that are full of color; distinguishing these colors helps fish hunt and avoid predators. Fish who live in deeper, darker waters don't need as much color vision; they tend to have more rods to help them see in low light rather than cones so they can distinguish multiple colors.
Unlike land animals, fish don't need eyelids to keep their eyes moist and shield them from bright light. Water tends to soften and refract light, helping protect fish eyes. Their eyes stay moist as the water moves over the surface of the fish's eyes as they swim or rest in a current.
The fish available for your aquarium should all be able to see, but not every fish in the ocean can. Those that live in the deepest areas of the ocean sometimes have no eyes or have eyes that don't work. The habitat is so deep that sunlight can't reach it, therefore no natural light exists. Some of these fish have eyes that can detect minute changes in light levels, following the phosphorescence of other deep-sea creatures, while others have no eyesight at all. They're built to survive in the extreme pressure created by the deep water, so they can't live in your aquarium. A few species adapted to live in underwater caves also lack eyesight.