FISH CAN DIE FROM SUNBURNS
When exposed to too much direct sunlight/UV light, just about any animal has the potential to get sunburned. Those that lack protective fur, such as fish, are most at risk. In fact, fish skin is more sensitive to sun damage than is human skin! Koi kept in ponds are particularly susceptible to sunburns, as are fish in outdoor farming facilities where overcrowding forces fish to the surface. The combination of shallow, clear water and lack of natural shading devices leaves these guys vulnerable to intense UV penetration, causing skin sores. The burns can be fatal if left untreated and unprotected. Source: Slate.com