Black sand magnetic??

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So I got some black sand from a friend with the tank I bought from him. He was using it as a FOWLR Tank. I heard about running a magnet through it and if it stick to it then its not safe? Well I disreguarded that since he had used it but when I put my powerhead in I dropped the magnet for it and sand stuck to it. I grabbed an actual magnet pushed it in the sand and it was covered and it wouldn't come off easily. Did I just waste 30 gallons of salt water that I put in there? Is the water safe to reuse? Is the sand unsafe? Help me out here.

 

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Here is a video of me putting the magnet in it...


[video=youtube;b-GuJEbBh-Q]https://www.youtube.com/watch?v=b-GuJEbBh-Q&list=UUYsfDnzWXDAkXK8RX1JiZVw[/video]