I've seen sump designs in which the downstream boundary consists of a weir followed by a sponge, floss, etc. What's the purpose of this sponge/floss? I'm guessing it's to catch small pieces of chaeto that pass through the weir. If this is the case, how necessary is it? If small pieces of chaeto get through, is it just an aesthetic issue relating to their getting into the DT? Is it something more problematic, such as clogging up the return pump? Thanks