What to fill empty space in filtration with?

In the back of my tank in the filtration there is some empty space (where foam pad-heater-bio balls used to be). what should i fill these areas with? should i use bags of carbon or should i add live rock rubble to them?? just wondering which one would be better to help clean with.

some of the areas i have partially filled with bagged carbon already, part of the ehater and some of the place where the foam pad used to go

thanks
steve

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