Seems there are many more posts addressing your skimmer question than your carbon question. I would run the skimmer 24/7. Keeping high oxygen levels being the primary reason.
As for carbon, it has always sparked debate on if it should be used or not, how often, and if it should be used passively or in a reactor. Carbon is generally safe. This is something you can determine to use or not and how often and you would probably be okay with it in any sense.
I always run carbon (6+ years) - a bag of carbon can last you a couple of months, so taking it out of your tank for a couple weeks a month doesn't make sense. It is a waste of carbon and your money. Instead you can extend the use in the tank and replace it every two months, in this amount of time the carbon will expire (bacteria will begin to colonize the carbon and essentially clog it up and it will stop working as it should), and essentially you are allowing the tank to run a short amount of time w/o activated carbon in it b/c the bacteria colonize have taken it over.
How Activated Carbon Works
Activated carbon absorbs substances from liquid. The word absorb is important here. When a material adsorbs something, it attaches to it by chemical attraction. The huge surface area of activated carbon gives it countless bonding sites. When certain chemicals pass next to the carbon surface, they attach to the surface and are trapped. Once all of the bonding sites are filled, activated carbon stops working. At that point you must replace it before it becomes a nitrate factory.
What Does Activated Carbon Take Out of the Water
Carbon is used to remove dissolved organic compounds (DOCs) from the water. The effect of DOCs cause the water to take on a yellowish appearance they can consist of complex organic toxins produced by the creatures in the tank.
Dissolved organic compounds( DOCs ) are the result of decomposition of uneaten food, digested food and dead inhabitants among other things. While small quantities of DOCs may be harmless, as a source of nitrogen compounds, at higher levels they can lead to water quality problems. Dissolved organic compounds contribute to the yellowing of water between water changes, reducing light penetration, an important issue when photosynthetic invertebrates and macroalgae are being maintained. Elevated DOC levels can also lead to outbreaks of nuisance algae, and there also appear to be correlations between elevated DOC levels, stress, and certain fish diseases as well.
Carbon is also used to remove fish medications from the water - Among the substances that activated carbon removes at least moderately well are acetone, alcohols (including isopropyl alcohol), antifreeze, benzene, chloramines, chloroform, chlorine, chlorophyll, citric acid, dyes, herbicides, hydrogen peroxide, insecticides, iodine, ketones, dissolved oils, organic acids, pesticides, phenols (including those that create unpleasant odors), radon, solvents and tar emulsions.
What Activated Carbon Does Not Remove
Activated carbon
does not remove all potential toxins or unwanted chemicals though, including the ammonia and nor does it substantially affect water hardness. Chemicals that activated carbon has little or no ability to remove include: ammonia, calcium, carbon dioxide, fluoride, lime, magnesium, nitrates, nitrites, phosphates, sodium, and iron. Lead and other heavy metals will only be removed by the use of a very specific type of carbon not normally used by aquarists. Therefore, activated carbon does not remove trace elements that benefit corals.
Pros and Cons
Pros to using carbon - Keeps the tank clean of DOCs and other toxins. Gets rid of the yellowish-green tinge water. In mixed coral tanks carbon helps to reduce allelopathy, it absorbs the toxins released by corals to harm other corals.
Cons to using carbon - Carbon dust has been proven by research to affect some fish lateral lines, essentially clogging the pores and causing fish head and lateral line erosion (HLLE). So if you buy carbon make sure you buy a high quality product. Carbon can leach phosphate, again buy a top quality carbon.
If You Use Carbon
If you use carbon, buy top quality carbon, always rinse it well before use to remove any dust, and don't leave it in your tank for too long, not only will bacteria colonize it and clog it, but it can become a dirt trap and add to nitrates.
References
http://www.hallman.org/filter/gac.html
http://reefkeeping.com/issues/2009-04/newbie/index.php
http://www.fishchannel.com/saltwate.../reef-aquarium-granular-activated-carbon.aspx
http://www.wetwebmedia.com/ca/volume_7/volume_7_1/carbon.html
http://science.howstuffworks.com/environmental/energy/question209.htm
http://www.advancedaquarist.com/blog/activated-carbon-affirmed-as-causative-agent-for-hlle-disease
http://www.advancedaquarist.com/blo...ted-in-inducing-head-and-lateral-line-erosion
http://www.coralmagazine-us.com/content/activated-carbon-hlle-smoking-gun-found
http://www.tandfonline.com/doi/abs/10.1080/08997659.2011.608608#.VGvKG8kSbBY
