The carbon isn't exhausted per say it's just gunked up from larger debris. Sponge pads help keep the carbon from moving and crumbling but it doesn't really act as a prefilter for the large gunk. Plus it's disposable toss it all out at the same time. I personally wouldn't try to go longer than 2 weeks regardless.
I'd personally would still combine them but just use less gfo since you are changing more frequently. So if you used 1 cup and wanted to change it out every month then just use .25 cups and change out weekly. Rox .8 carbon is good stuff a little goes a long way, if you using more than 1/4 cup at a time for your tank then your wasting carbon. Capacity still stays the same and its really better for your tank to do less and change more frequently for stability/consistancy.
I'd personally would still combine them but just use less gfo since you are changing more frequently. So if you used 1 cup and wanted to change it out every month then just use .25 cups and change out weekly. Rox .8 carbon is good stuff a little goes a long way, if you using more than 1/4 cup at a time for your tank then your wasting carbon. Capacity still stays the same and its really better for your tank to do less and change more frequently for stability/consistancy.
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