Black powder?

[MikeP]

When I was first setting up my tank I was in a LFS and one of the staff was telling me about something called black powder and how awsome it was. But continued to say they don't sell it but if I find some I should buy it. Well I found another store that sells it so should I get some? What will it do for my tank? I haven't actually read the container so I don't even know what the manufaturer claims it will do. This lady talked like it was one of the best things you can do for your tank though.

Any info would be great
Thanks