Notes

Ornamental fish – such as goldfish – have even better vision than humans. The reason? Their eyes have additional photoreceptors for colour vision, enabling them to see ultraviolet light, which gives them a huge advantage when underwater.
Unlike us humans, goldfish can make out the tiniest of details in clear water that’s rich in UV light.
Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens.
Fish retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarized light.
Fish can see clearly underwater because their eyes are convex enabling them to focus the light. The fish eye is larger because so that more light can be refracted. When we open our eyes, our corneas are in contact with the water. The contact makes our vision fuzzy.
Rods “see” light, while cones provide color and detail. Like us, fish cannot see color at night, or other low-light conditions. A fish's eye is adapted to better see movement and contrast at the expense of detail. The cones in a human eye are densely packed around the retina, allowing for better detail.
We see what is called the visible spectrum. The actual colors within the visible spectrum are determined by the wavelengths of the light: the longer wavelengths are red and orange; the shorter wavelengths are green, blue, and violet. Many fish, however, can see colors that we do not, including ultraviolet.