most surface dwelling and upper water fishes do get lots of sunlight in the wild.
This is not the case. Maybe in northern Australia, but not South America where most of our fish come from. An excerpt from an article I authored elsewhere:
The Brightness of the Tank Light
“Daylight” can be vastly different for different species of fish, depending upon their natural habitats. Most aquarium fish are forest fish, living in rivers, streams and creeks, ponds and small lakes, and even ditches and swamps. Some of these watercourses are permanently shaded by the forest canopy or overhanging marginal vegetation; some are filled with branches and sunken bogwood; some watercourses are white-water, full of suspended particulate matter that “soaks up” the light and keeps the water murky and dimly lit; watercourses in full sunlight usually have a thick mat of floating plants, and depending upon the position of the sun in the sky, much of its light may be reflected off the surface. In most areas of the tropics, the rivers and streams rise and flood the surrounding forest for half the year, and it is during this period that the fish move into the even darker forest waters to spawn. For fish living in all of these environments, bright light is something they seldom—and in some cases never—experience, or from which they prefer to retreat given the opportunity. It is no surprise that the fish in the brightly-lit watercourses are almost always found at the edge under overhanging vegetation, branches and outcrops, or floating vegetation.
Programmed by nature over millions of years for such dimly-lit environments, the eyes of a fish are designed to capture the maximum amount of light. This allows the fish to have some degree of vision even in dimly-lit water. But when placed in brighter light, the fish will naturally look for cover in order to escape from that light. Aquarists can readily see this in the aquarium; many forest fish when given the option clearly prefer shaded areas. Baensch & Riehl (1987) called it a “light phobia” in characins. And it impacts the fish’s colouration too.
Fish (and amphibians) possess specialized cells called melanophores that contain hundreds of melanin-filled pigment granules, termed melanosomes. The sole function of these cells is pigment aggregation in the center of the cell or dispersion throughout the cytoplasm, thus altering the shade of colour. The melanophore cells are themselves light sensitive and respond directly to light by melanosome translocations, which is why the fish’s colouration pales during total darkness. Pencilfish in the genus Nannostomus have a diurnal patterning; at night the lateral lines break up into dashes, and the fish pales. This has been noted to occur even in blind fish, which shows that it is not sight that tells the fish it is getting dark, but the increase in darkness probably coupled with the circadian rhythm that sets in motion the natural response of the fish to darkness.
But the fish can also adjust the melanosomes voluntarily in response to environmental stimuli, stress, and social interactions. Fish in the store tank often appear pale, or after being netted and bagged will often be without any colour. Fish kept in brightly-lit tanks will frequently exhibit much less intense colouration. In all these cases, this “washed out” appearance is a natural response caused by high stress. Floating plants are one of the best ways to provide shade, along with using the minimum intensity of light necessary; the reward to the aquarist will be brighter colouration and increased interaction between the fish—and that spells healthier fish.