with the amount of turnover your filters going to have on such a small tank Garuf, filling it right up with media is probably a waste of money.
As Ed said on a different thread.. a tank is only going to produce so much NH4, so only a culture of a certain size is going to form anyway.. with a tank so small, on a filter so large, whats the point in filling it right up with expensive media if the culture might only use say 25% of the surface area available anyway?
Id fill it up with polishing pads and other things that are cheap as chips, mainly to slow the thing down!! use 1 tray for biomedia.. you have no need to use any more than that.