I understand you called inquiring about potential sources of arsenic (As) in your water samples. Unfortunately, arsenic is a common contaminant. If you see it in your freshly mixed saltwater, the likely source is either your RO/DI water or the salt mix itself. We routinely see both, so I suggest sending in a test kit with your RO/DI water. If you do not see arsenic contamination there, then the only other source would be the salt mix, which we have seen in the past from some salt vendors.
The situation is more complicated when you find arsenic in aquarium water samples. Given that it is such a common contaminant, there can be many possible sources. However, it is usually from a product added to the system in more significant amounts, such as two-part solutions, alkalinity correction solutions, etc.
Hopefully, this is helpful. Feel free to reach out with any other questions.