Tropic Marin is a great salt, always highly reccomended. My guess is that, like you said, it's found at such small numbers that it is no harm at all. I did a quick google on Tropic Marin and it says it contains 70 trace elements found in natural sea water. Then you said it lists iron, arsenic, and copper as ingredients on the side of the box.
Kinda makes me wonder what prolonged use of Tropic Marin would do to a system. Would it cause an increase in copper levels over time? It's interesting. I looked up the Seachem salt that I'm using, and it says it contains .0003 ppm
(I think ppm
I beleive it was Shimek who wrote an article on the toxicity of synthetic sea salts we use for our tanks.
No answer I know, sorry. It's interesting.
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