"Thiaminase (an enzyme that metabolizes or breaks down thiamine) is also considered a problem that may be a problem in fish afflicted with HITH. Thiaminase is introduced via feeding fish such as Goldfish/Fathead Minnows, or via bacteria. Thaimase breaks down thiamine, and thiamine is an important vitamin necessary for carbohydrate metabolism and normal neural activity. If "feeders" (goldfish sold for the purpose of feeding other fish) are fed as a large portion of your fish’s diet, it likely will develop a thiamine deficiency.
Although many dry prepared foods contain fish flesh that also contains Thiaminase, however the process required to prepare a dry food/pellet likely destroys this enzyme." This is taken from
http://www.aquarium-pond-answers.com/2008/02/hole-in-head.html this in-fact is not the only site i have found to mention this. he dose say that he has not done to much scientific research on it but he believes with his over 25 years in the fish business and research he dose believe to to be a cause.