Here’s an excerpt from an out-of-print but still very informative book titled Aquarium Ecology. It discusses why two protein skimmers on the same system will probably not perform as well as one larger skimmer. The translation is a bit rough and a few of the sentences aren’t entirely clear, but it’s still an interesting topic which some may find useful.
Behavior of two or more skimmers in an aquarium
Protein skimmers are interfacial adsorbers and operate by means of buoyancy. If they had ideal properties with an efficiency of 100 per cent, they could keep the cellulose and protein level in an aquarium at almost 0.0 mg/l. In reality, however, the values range around 0.4 mg/l, which is an excellent value in view of the varying surface tension of the tank water (during feeding, for example) which has an influence on the skimming performance.
After all, an ecosystem like a reef aquarium also needs a certain quantity of protein for its food web. Apart from the fact that aeration is out of place with skimming in an aquarium because it binds proteins, the question actually is whether it Is sensible to use two or more skimmers in a closed system (aquarium).
Many years of observation have revealed that only the use of skimmers of the same type, i.e. of same make, model and performance, is sensible. Even in this case differences in the skimming amount will occur as a consequence of slight manufacturing differences or settings of the air quantity. A large skimmer alongside a smaller skimmer will tie up the latter.
The following hypothesis can be made to explain this phenomenon: The foam discharge zones of skimmers behave towards the protein-containing surfaces of an aquarium comparable to the resistance towards the discharge of the water The ideal skimmer thus would have an extremely low resistance. A smaller skimmer alongside a larger skimmer thus cannot be a recommended choice because almost all protein will be removed by the large one. This is especially true considering that larger skimmers are less sensitive (speaking generally) to variations of the surface tension and thus operate more stably.
If, however “the stable door was locked after the horse has bolted” and two skimmers of different performances have already been fitted, then a distance between the two units as large as possible Is recommended. The removal of the protein particles by the skimmer with the larger performance has less effect then on the other skimmer. Of course a large distance between the units Is also sensible even in cases where the two skimmers are of the same type or have identical efficiencies.
Aquarium Ecology. Pages. 43-44. TUNZE Aquarientechnik GmbH, D-82374 Penzberg, Fed. Rep of Germany