Here is Seachems response, I was curious as well....
Quote:
The black specs are just excess Prime. Prime is as concentrated as we can make it. Sometimes excess will precipitate out and that is what the black specs are.
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FYI, Prime is bisulfite and hydrosulfite salts. Although Seachem was not specific, I would surmise the black specs are a precipitate of sulphites.
Cheers
Steve