Way to go e-cat. If you look at your tank thru the LONG side of the tank your water might look more clear/yellow than crystal clear. Take a white 5 gal
bucket and fill it with tank water and you will definatly see a yellow tint compaired to a fresh 5 gal
bucket of saltwater.
The tank looks clear because all those disolved solids are diluted in the water. (this is why we say larger tanks are better). When the skimmer concentrates all those solids into the skimmer cup you end up with that dark brown/black sludge.
Think of your skimmer as a kind of home sewage processing plant for your tank.