When to add gfo and carbon?

If it were me I would get a Hanna PO4 checker and only run GFO as needed. Carbon helps improve water clarity to I like to run it all the time.

Running your skimmer is also fine as they usually take a few weeks to "break in" anyway.

 

if youre cycling with an occasional sprinke of fish food i would at least check po4 at least once to twice a week and treat as necessary since you obviously dont have any fish in there to consume the food so your po4 may climb. as far as carbon and your skimmer... i personally would run both right from the get go and get used to operating your tank like you would if you had it stocked...for me cycling a tank was a good practice period to get used to changing filter socks and some of the other maintenance to your tank.

what brand of carbon are you going to be using? my personal preference is seachem's matrix carbon.

skimmers will go hay-wire fromt he start and if your was brand new a good thorough cleaning of it before use is always suggested as their is a lubricant or other substance that may remain on the inside of your skimmer from the manufacturer. a good cleaning or rinsing of it should help it break in quicker.

 

You sound like you're sitting pretty well prepared...just keep tabs on the phosphates. Not sure if it would hurt or not to just start gfo now to keep your levels from even going up or if you should wait until there is a detectable undesirable level of po4 before starting gfo treatment.