Some recent articles in Advanced Aquarist by Ken Feldman et al have looked at the performance of skimmers and the constituents of skimmate. There are some surprising results. In a nutshell, skimmers are only capable of removing about 25% of total organic carbon. That may be due to many compounds not being attracted to the air/water interface. It also turns out that skimmate is, surprisingly, made up largely of inorganic compounds. That may mean that skimmers are mostly removing microorganisms such as diatoms, not free organic molecules. http://www.advancedaquarist.com/2010/2/aafeature http://www.advancedaquarist.com/2010/1/aafeature http://www.advancedaquarist.com/2009/1/aafeature2 This is all making me wonder if a skimmer really provides much benefit. It seems like if there are organic compounds that really need to be removed, something like GAC will likely be necessary since a skimmer only removes a small percentage of them. Skimmers may help with nutrient control because they remove microorganisms, but growing and harvesting macroalgae may be just as (or possibly more) effective at that. So if you're running carbon and growing chaeto, does a skimmer really provide any benefit? I'm curious to hear opinions on this. Also, if you've tried running your tank both with and without a skimmer, what differences did you see?