sure
First level is prefilter and different types of filter pads from bio,chemical,mechanical to get the big stuff, second level is for mechanical and is filled with cruched lava rock and ceramic rings, third level is sponges for bio to build up bacteria...
I am not really a great person to explin this, but that is what i am told they do..
all I know is that my tank is crystal clear all the time and all water parems are perfect...