Depends how it's used, but generally no.
If it's part of a pump or something and is in constant motion then you're probably fine (ie bearing in a pump), but using stainless steel structure or fasteners inside your tank is not a good idea. It will eventually rust and release potentially deadly oxides(?)* into the water. It will also be hard to clean out after, as flecks of rusting stainless steel can release, and hide in the substrate rusting away.
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