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Why are frozen fish bad for turtles?
Is it the process of fish meat turning into 'ice' that is causing something negative,
or is it that manufacturers add something to fishes prior to freezing?
Can I just buy a raw fish and freeze it myself?

Freezing does kill any potential parasites. It is believed that frozenfish are likely to contain more Thiaminase, an enzyme that destroys Vitamin B1 (thiamin). Vegetables and prey high thiaminase enzymes would break down thiamin, resulting in a Vitamin B1 deficiency.

So fish should just be a rare treat

I'm not sure where you live but I have read that all dead fish sold in the US has Ethoxyquin added as a preservative and if I remember right it causes cancer in animals and has been banned in most human foods.

What if I buy fresh fish and then freeze it myself?