It's not wire strands it's still coal slag,more like little "glass" tubes, I haven't really found that fish have a problem with it even with fish that take the substrate into their mouth and spit it out, those pieces just happened to be narrow enough to get through their screens for size imo, you would need a different stage than screening into different grits to remove those "wire" pieces.
Maybe some coal slag has actual bits of wire but I've never personally seen it.