It's very fine. I have it in a 10g shrimp tank & added some black Estes marine sand to keep the dustiness down. I still suck up a bit each WC using a small gravel vac. I haven't kept eels so I don't know if they're vigorous diggers but you may want to use a prefilter (AC sponge over intake or similar)
I attempted to rinse it before using it but that was too difficult. I wrapped floss over the intake until the black cloud subsided, a few days with a very small HOB. The plants seem to like it though any replanting kicks up some dust, fairly minor. I'm not sure how much it might get blown around by a bigger filter.