Thats right , not visible when you are just looking from outside of the tank.
The moment you lift it and squish the sponge in a bucket (with the original tank water of course) then only you will know how dirty the sponge filter is.
I like the water fall sound but the problem with me is that I always have my water level in my tank at the most maximum .. so the sound of water dropping from the HOB is not that loud if any ...
Actually I am tempted to buy fish online but then again didnt have the hearts thinking about those long hours cramp in such a small plastic bags ... I better off head to my LFS and transport em myself ...