The eyes can become cloudy due to extra slime (mucus) being formed on the outside as a result of skin irritation caused by parasites or poor water quality. (Ill. 124)
The eyes can become opaque as a result of damage (e.g., aggression) (Ill. 126)
or internal moisture accumulation (with bacterial infection) (Ill. 121, 123)
The eyes can protrude, also called exophthalmia or pop-eye, in which case they are usually opaque and filled with moisture. (Ill. 121-123)
Bulging eyes also occurs with FishMB. (Ill. 120+121)
Protruding eyes are often associated with swollen bellies (see below) and sunken eyes are encountered in extremely emaciated or exhausted fish (see below).
Extreme exhaustion caused by stress or disease may lead to sunken eyes (endophthalmia). (Ill. 128-130)
Aggression, cannibalism or inbreeding may result in permanently damaged or even absent eyes. (Ill. 126+127)
Sometimes there are parasites in the eyes (such as worm infections in wild-caught fish) or around the eyes (such as parasitic Copepods). (Ill. 131)
Tip: Eye problems can have a wide variety of causes.