Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
New Flourite is pretty dusty, and you can never fully wash it all out. The fine dust may be leaching some minerals out. I've not checked GH specifically, but have seen some evidence that new Flourite Black can initially leach out a good bit of manganese.
If this is the case, it should resolve within a week or two.