Actually it's thiaminase that can cause one of the problems with goldfish. Thiaminase is an enzyme that eats up and depletes the turtles of thiamine which is one of the B vitamins. The thiaminase is higher in fish that are frozen.
Goiters are caused by the lack of iodine in the diet, that's why table salt has iodine added to it.
Another reason why goldfish aren't the best choice is because they are a fatty fish. Guppies and minnows are a better choice for turtles to eat because they're not so full of fat.
You have to be careful with mosquito fish too because they tend to harbor a lot of parasites.
I feed my turtles live feeder fish so I do have them parasite tested at least once a year.
>>Yes, I heard the same thing...since you brought it up. People say it has too much thaimin, or vitamin A with the combination of chemicals to keep the fish from dying in crowded living condition. I'm not sure if thaimin would do anything...because we and animals need it. Animals and people that don't get enough thaimin will develop goiter, and become overweight, lathargic, and then develop diabete as a result, and vitamin is very important for development of good eye sight which is very important. A severe deficiency in vitamin A can cause blindness. The key is moderation with anything. Although, I'm not sure about the chemicals, used to help the fish stay alive, can cause any harmful side-affects.
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PHRatz