Caper, I got Raphael from Phoenix, the video was taken in my tank.
Its <4mm pumice. Its pretty much sand, just heavier and doesn't get stirred up every time something moves.
Why should they only be on sand?
I just pulled the lid off mine, i assume they are similar. there is a rubber plug in the side of the head that i dont know what it does. is yours still there?
I have a cf700
when mine doesnt start i tip it on its side, so the taps are pointing down. this has not failed to make it work for me. once it starts going stand it up slowly to let the bubbles out slowly