I always though that both the hole in the head and lateral line diseases are associated with poor nutrition. Properly fed fish might be less suspectible to any protozoan/infections. Feeding your fish a strict died of gold fish make for poor nutrition especially in aquarium environment.
In nature the predators that feed constantly on "meat" also consume plant matter that is found in the fish they just swallowd. In aquarium enviroment gold fish purchased are so underfed (too keep water quality in check) that there is not much nutrition in them.
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BAS - Educating Aquarists Since 1911.
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