HCA: Hermit Crab Association

i have a question that there are many reptile & turtle foods in the pet shops , but they are good for land hermit crabs.

I have not tried any of these food, I do not know it is harmful or not as reptile, turtle and land hermit crabs are different type of animal.

But a Tawanese people said that he used turtle "Calcium powder" to provide calcium need for his crab and reptile "vitamins".......is is good or not??

Calcium powder for reptiles is fine to use, and I will sometimes use reptile vitamins for the hermies, too. But I wouldn't feed them most commercial turtle or reptile foods, because most of them have harmful preservatives, such as ethoxyquin. If they don't contain any harmful ingredients, I think you could give turtle or reptile food to the hermies sometimes, as part of a balanced and varied diet.

What other kinds of harmful ingredients should be avoided?

Most of the commercial turtle and herp foods contain waste products because it's cheap. "Plant and animal by-products" (I think that's the term) is just a nice way of saying: we put anything in that food that we couldn't use for anything else.