How many gallons is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
Does the patch have any redness to it. Does it look like bleaching beneath the skin. Does the patch look fluffy.
Could the fish have injured himself on something in the tank.
Any signs of flicking and rubbing or excess mucas.
Sorry read it wrong so the patch is on there eyes it's self.
Any white specs behind the lens of the eye.
Does the white patch look fluffy.
Could the fish have injured his eye on something in the tank.