It could decrease. It's a hold up from freshwater that a "filter" is mandatory.
Your rock is the biological, and your skimmer does the rest. It's known as the berlin method, it used to be one of many when trickle filters, etc were popular, but now it's pretty much standard.
Even tanks with sumps don't really have "filters". A sump is just somewhere to put the heater, skimmer and maybe a filter sock, but that's just for mechanical filtration, and that's not required at all for tank health.