Betta fish turn pale when they are affected by a fungal infection. White patches of fungus can grow on betta fish, causing them to look paler than usual, according to Betta Fish City.
According to PetMD, fungal infections in fish cause white or light gray patches to appear on the fish's body. These growths can affect the scales, fins and gills of the fish, and they can also occur around the eyes. According to Betta Fish City, other symptoms of fungal infections in betta fish include lethargy, sometimes accompanied by a lack of appetite. Fungal infections can be treated by changing the water frequently and adding antifungal drops to the tank.