Conclusions
Many factors contribute to the "value" of a skimmer to an aquarist, including quality of construction, size, footprint, noise level, ease of cleaning, energy efficiency of the pump, and of course, the ability to remove organic waste from aquarium water. Our data show that there are not compelling or remarkably large differences in measurable skimmer TOC removal metrics among the seven skimmers tested, although the Reef Octopus 150 consistently underperformed compared to the other skimmers. However, in the larger picture, it is equally apparent that if an aquarist runs a skimmer continuously (24/7), then any of the skimmers tested would perform adequately in terms of rate of TOC removal; the only practical differences might involve the frequency of skimmer cup cleaning. A perhaps more interesting observation to emerge from these skimmer studies involves not the rate of TOC removal, but rather the amount of TOC removed. None of the skimmers tested removed more than 35% of the extant TOC, leading to the conclusion that bubbles are really not a very effective medium for organic nutrient removal. If fact, the presence of refractory, or unskimmable, TOC, coupled with the likelihood that endogenous TOC consumers (bacteria, among others) also do not remove all of the TOC present (cf. Fig. 4), suggest that in an operational sense, TOC can be categorized as follows:
TOC that a skimmer removes
TOC that a skimmer does not remove
TOC that is consumed by microbes
TOC that is not consumed by microbes
TOC that is (indirectly or directly) harmful to tank livestock
TOC that is not harmful to tank livestock
The last two categories must be included as a result of recent work of Forest Rohwer (See the January 2009 Advanced Aquarist article for a discussion), and they really highlight why an aquarist might be concerned with rising tank TOC levels. Of course, there will be much overlap between these categories. Ultimately, the crucial question for sustaining aquarium livestock health over the long term is, "How much of the harmful TOC (#5) is removed by either biological consumption or by skimming?" That question remains unanswered at present.
The results to date on protein skimming as a means of aquarium water remediation form a consistent picture that is at odds with some of the cherished dogma in the marine husbandry area. According to the data presented in this and the earlier paper (Advanced Aquarist, January 2009), protein skimmers appear to have a much larger variation in their prices than they do in their ability to remove TOC from aquarium water. Recent design innovations like bubble plates, conical sides, or pinwheel impellers do not seem to impact significantly on either rate of TOC removal or amount of TOC removed, at least for the skimmers tested. Thus, skimmer manufacturer claims about enhanced organic removal capabilities should be met with skepticism in the absence of compelling and quantitative TOC removal data.
