This is too cool. The multiple benefits of fish poop
http://www.comcast.net/articles/news-science/20090115/SCI.Fish.Poop/
Well now THERE is somethig to talk about at dinner tonight.
Check out the table about half way down on this article it talks about the food you feed your fish contributes to the calc, mag of a tank (over time) http://www.advancedaquarist.com/issues/oct2003/chem.htm.
It all depends on what they are eating!