The visual system of fishes is highly energetically expensive, both while processing information and at rest. Indeed, fishes living in permanent darkness often have visual structures that are reduced or absent altogether. Many other fishes experience periods of prolonged darkness throughout life (e.g. during estivation), yet the impact on their visual system is unclear. We used an amphibious killifish (Kryptolebias marmoratus) that occupies dark terrestrial habitats (e.g. rotting logs) during seasonal droughts to test the hypotheses that 1) exposure to prolonged darkness during air-exposure diminishes visual acuity and in turn, impairs hunting ability upon return to water, and 2) diminished visual acuity results from a reduction the size of the optic tectum (OT). We performed 3 week acclimations with a 2×2 factorial design, in which fish were either acclimated to a 12:12 or 0:24 light:dark photoperiod in water or in air. We then measured the optokinetic reflex in water as an estimate of visual acuity, hunting performance (i.e. the ability to leap out of water and capture terrestrial prey), and the size of the OT. In support of our hypothesis, we found that fish held in water had reduced visual acuity when acclimated to the dark, although fish held in air had poorer vision when returned to water regardless of photoperiod. The hunting performance of K. marmoratus followed a similar trend, suggesting that good vision is important for effectively hunting terrestrial prey. Changes in visual acuity did not result from an altered OT size because the OT did not differ in size between groups. Overall, our findings indicate that periods of prolonged darkness and air-exposure can impair vision in amphibious fish, as well as their ability to perform important visually demanding tasks.