I'm not familiar with the Juwel filters, so I can't tell you exactly what your black spounge is off the top of my head. If it is like a pouch with chips of carbon in it (mine are about the size of one grain of aquarium salt), or some how holds them, then it is a carbon filter. If it is literally a black spoung, with nothing inside of it, that is where your bio spounge, or where the beneficial bacteria live.
Platyfreak was warning you to be careful with whatever ich meds you use, because scaleless fish (loaches) can be very sensitive to meds, and some medicines need to be used at half strength with the scaless fish. Just follow the directions on your box.