Ive done the magnet test with the black CaribSea Tahitian Moon. It stuck to the magnet a bit,(a Great White by Algae Free, Very Very Very strong magnet!!!) But Ive also used the Tahitian Moon in a reef with no ill affects, corals did well, fish were healthy, no algae problems, macro algae grew well. Id use it again! Actually Im setting up a Nano Reef with the Tahitian Moon right now... I also use it in freshwater tanks with Arrowanas, Stingrays, and Vampire Tetras(Rhaphiodon vulpinus), as well as Discus.
Never had a problem with it...