In the mid 1970’s, Peter Wilken published a book entitled The Saltwater Aquarium for Tropical Marine Invertebrates. This landmark publication was one of the first to introduce a more natural approach to keeping sensitive invertebrates in aquaria. As time moved on, the “Invertebrate Aquaria” was renamed the “Reef Aquarium”, and the filtering techniques described by Mr. Wilkens later became known as the Berlin Method.

The Berlin Method is simple. It includes only three items to do all aquarium filtration: Live Rock, an overflow with mechanical filter, and a large Protein Skimmer.

Each of these items work together to complete the filtration picture. The Live Rock performs the function of a biological filter and provides nitrate reduction. The mechanical filter removes detritus and algae that naturally accumulates due to the metabolism of the invertebrates. And the protein skimmer removes nitrates, phosphates, and DOCs (dissolved organic carbons).

It’s been over thirty years since the original publication, and no one has come up with a better way to filter a reef tank. Sure, we now have Sulfur Filters, Phosphate Reactors, and a slew of other equipment at our disposal. But all this extra junk just boosts the power of the original Berlin design.

Somewhere along the way, many hobbyists decided that the mechanical filter in the Berlin Method was not necessary. Some go as far as stating it’s downright detrimental in keeping corals. The reason you most often hear is that mechanical filters remove pods and zooplankton that otherwise would feed the corals in the tank.

Quite frankly, this is bullshit. And here’s why.

No one can grow or sustain zooplankton in a reef aquarium. Period. Zooplankton is an “organism soup” of micro life that lives in the water column, most of which remain in a planktonic state and “free float” for their entire lives. Reef aquariums cannot produce the immense quantities of phytoplankton and other micron-size live food particles to sustain any populations of zooplankton. On the real reef, zooplankton is abundant and is consumed every night by corals and fish larvae. Unfortunately, these microorganisms simply don’t exist and never will in your aquarium. So a mechanical filter cannot remove zooplankton, because there isn’t any to remove.

Secondly, there are the Pods. Copepods, Amphipods, and Isopods can easily reproduce in large numbers in a reef aquarium. But the pods that grow well in our tanks are all benthic, meaning that they are bottom-dwelling creatures that don’t live in the water column. Have you ever experienced your front glass being covered by little white moving dots? These are Isopods. But you may have also noticed that the water didn’t look like it was snowing with them- only surfaces like the front glass and the Live Rock were populated. The don’t swim- they walk. The only way these little guys will feed your corals is if they accidentally crawl into a polyp. A mechanical filter will not remove Isopods unless they lose their footing and get swept into your overflow.

The larger Amphipods are quite common and can be seen crawling around on the Live Rock. As far as pods go, these guys are pretty big at around 5000 microns (about an 1/8-inch) as adults. The have a hard shell with a series of legs underneath and resemble a very tiny shrimp. They are an excellent food for Mandarins and other bottom feeding fish. They are however, too large to feed SPS polyps and too small to feed LPS corals. They guys would have to walk into the mouths of LPS corals to get eaten, which is not going to happen. These pods live everywhere- both in the sump/refugium and in the main tank. Unfortunately, most are too big to survive the trip through the return pump back to the main tank. Again, a mechanical filter might trap a few that get swept away by the current, but not enough to make a difference.

I decided to examine the contents of a used mechanical filter under a microscope. The filter sock I examined was in service for 3 weeks on a 60 gallon SPS reef tank. It’s pore size is 100 microns and composed of spun polypropylene. All of water flowing from the main tank through the overflow passed 100% through this filter. Within the 3 week time period, it was so clogged up that water would not freely pass through it anymore.

I took several scrapings of the material on the inside of the bag and placed it on well slides. The first thing I saw were diatoms (aka brown algae). Zillions and zillions of them. In the photo above, you can see the brown color of the spent filter sock caused by all the diatoms. Surprisingly, there is very little brown algae spots or brown haze anywhere on the glass panels in the aquarium.

Probably 90% of the material on the filter sock was brown algae. But there were also green microalgae and small strands of what appeared to be hair algae in great abundance. As for life forms, there were some very tiny (2-4 micron) single celled organisms swimming around. I don’t believe the 100 micron filter sock had captured these guys. They were just swimming in the droplets of water on both side of the filter material.

Finally, lots of detritus chunks. Many were entangled in a stringy algae mess, some pieces were clearly encased in a bacterial slime. Others looked like small rocks. At least this time, I did not find even one pod in the half-dozen or so scrapings I examined.

So what does this all mean?

While the Berlin Method touts the mechanical filter as a detritus removal mechanism, my own experimentation indicates that it’s main function is that of an algae filter. Both substances are equally harmful to the health of saltwater aquariums. Detritus robs the tank of oxygen and lowers PH and ORP. Algae smothers out invertebrates and causes severe water problems. While many things contribute to algae outbreaks, my opinion is that using a mechanical filter will diminish the occurrence of film algae, and prevent the spread of undesirable macro and hair algae. Besides, once you start using a mechanical filter, you will have the cleanest sump in town.

If you don’t already employ a mechanical filter in your reef tank, give it a try. I believe it is an important component of both algae and water quality control.