Peter Pan wrote:
I never heard of this but;

It's much more common in Europe (from what I read).

Here's one:
http://www.marinedepot.com/aquarium_...denitrator.asp

They are also used for public water supplies to remove nitrates from
fresh water.

In sal****er, the reactions aren't well understood, but they appear to
work well, but they may raise PH due to proton release so they are back
ended with a calcium carbonate chamber (which in theory will release Ca
ions).

They are amazingly expensive for a tube of plastic with separated
sections for elemental sulfur and calcium carbonate, but I was planning
to build my own.

--Kurt