The coal slag and the iron silicate should be the same thing, just different names. The link posted above goes to us minerals which makes the coal slag and talks about the iron silicate, which is the same thing. I use it in my 3 tanks and have healthy plants and fish and no complaints what so ever. Just be sure to rinse very very thoroughly, i sprayed mine in a tub with a jet setting on a garden hose sprayer and probably rinsed it about 8 times, basically until you notice no notable difference in a rinse. Other than that I love the color, the ease of use, availability, and the price.