How well can aquarium fish see?

Much depends on the natural environment where they occur. Among those species with the most developed eyesight is the so-called four-eyed fish (Anableps anableps) which is found in parts of Central and South America. These fish inhabit quite shallow, clear water and spend much of their time close to the surface.
Four-eyed fish
Each eye is actually divided into two parts, with one part being developed to see well in air, while the other operates underwater. Each segment of the eye operates independently, so they can scour the water looking for possible food, while also being on the look-out for possible danger from above.

By way of contrast, the blind cave fish (Astyanax fasciatus mexicanus) has no vision at all, as its name suggests.Mexican blind cave fish Although these fish actually hatch with functional eyes, skin soon grows over them, so they lose their sight at an early age. This is because vision is of no value in the dark Mexican caves which they inhabit - they have also lost their body pigmentation, and so appear white.

Blind cave fish have become isolated here and have adapted to this new environment, whereas their normally-sighted and colored relatives are to be found in nearby rivers. (Photo shown here courtesy of JohnstonDJ).