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you would have to rinse it rediculously well to not get any black smakey look in the tank. I did it with fish in the tank, the fish were fine and a took only about a few hours to clear out and thats without rinsing it well.
It could be if you dont really rinse the sand. Make sure you clean it really good or it will keep your water cloudy for a few days. I know this from experience. I thought I cleaned it pretty good...then to find out I have not..lol
get a bucket dunk sand in, fill with water, stir and then dump out water, repeat this until water comes out clear, always stir vigerously (or however it is you spell that)