Sponges can be a pain in the butt. If I were you I would remove the sponge below the output of your skimmer. It might clog or stop water from returning to your tank and cause an overflow. The other sponge I would clean once a week. As far as the Wet/Dry, if you are keeping fish only that would be okay. If you plan to keep corals then they are old school and I would remove them slowly from your system...a layer or two at a time until they are all gone. Your skimmer is your main flilter in a coral or reef setup.