I'm curious how long people have been feeding frozen fish fillets? This seems like a relatively new occurrence that most people have not heard of before coming to this forum. I'm not saying it's bad... I'm assuming we really don't know what kind of nutrition is truly optimal for a garter or how to get that nutrition into them. We're all just doing the best we can with the information we have.
A vitamin supplement, an effort on our parts to find filets that still have bones and skin, and a diet that varies to some degree and includes rodents is a safe bet