Dr. Mark Bethea - May 11,1997

Hello, my name is Mark D. Bethea, Ph.D. of the Dr. MDB's reef page. Today I will talk aboutbiological filtration and give some pros and cons of the different types. I have been keepingmarine and freshwater aquariums for over 15 years and have a reef web site located at http://idt.net/~mbeth19/reef.html.

Filtration

There are three main types of filtration: biological, mechanical and chemical and all of themfunction by allowing water to flow through or over the substrate of filter media. The greater thesurface area, the greater the filtration potential of the media. I will talk about the first type offiltration mentioned, biological filtration and give an overview along with pros and cons of thetwo most common of biological filtration used in the home saltwater aquarium today. Thesethree types are Undergravel Filtration, Wet/Dry Filtration.

Biological Filtration: Biological filtration utilizes living organisms to remove or transform toxiccompounds generated within an aquarium system. Colonies of nitrifying bacteria living on asubstrate oxidize toxic metabolic waste, ammonia and nitrate into harmless nitrates. This processis call nitrification. In the home aquarium, biological filtration is a natural means of removingtoxic ammonia-based wastes from the aquarium. Ammonia is toxic to fishes and invertebratesand if it is not removed efficiently and converted to less harmful substances, all aquatic life will suffer and soon die. Fishes and invertebrates excrete ammonia which is added to the ammoniaproduced by bacteria working on other waste materials in the aquarium such as dead organismsand uneaten food.

Aerobic (oxygen-loving) bacteria such as Nitrosomonas sp. Convert ammonia to nitrite which isslightly less toxic than ammonia but still harmful to fish and invertebrates. Nitrobacter sp.Convert the nitrite into nitrate which is a much safer substance but it can still be a problem. Theoverall goal is to keep your nitrate levels as low as possible by successfully managing thenitrogen cycle.

The nitrogen cycle starts with the fish or coral ingesting food. Next, the waste products of thefish or coral are decomposed by bacteria and converted to ammonia. Then, nitrite bacteria(nitrosomonas) convert the ammonia into nitrites. Finally, the nitrate bacteria (nitrobacter)converts the nitrite into nitrates and the process starts over again.

I will now discuss the two most common types of biological filtration used in the saltwaterhobby today:

Undergravel Filtration:This is still the most popular type of biological filtration method to date. An undergravel filter consists of a perforated plate, covered by a 2.5 to 4in. layer of substrate. Most undergravel filters are powered by airlift tubes that are not always efficient. An airlift'sflow volume is directly influenced by the flow to the airlift tube. When air stones becomeclogged, the flow decreases and the biological bed suffers. One solution to using air stone(s) andair pump(s) is the use of power heads for water flow through the airlift tubes. This flow is muchmore constant and enables a more accurate flow throughout the undergravel filter bed. There is adrawback to using powerheads since they can be come partially blocked by waste and gravelfrom the undergravel filter media. A reduction of flow can be very dangerous and result in aconsiderable loss of biological filtration efficiency, resulting in reduced oxygen/carbon dioxideexchange. The only advantage that an undergravel has is in terms of price. You can set up atypical undergravel filter system for less than $30 for a 55 gallon tank. There are severaldrawbacks to such a system. Over time, undergravel filters can become clogged with detritusand waste and this causes ammonia surges which can be deadly to your tank's inhabitants. Whenusing air pumps, the pumps do not usually run at maximum efficiency all of the time and this canresult in an uneven flow through the airlift tubes. When using powerheads, the powerheadsthemselves may become clogged with debris and this also reduces the flow rate. Undergravel filters are also high maintenance requiring constant cleaning of the substrate (which can cause anammonia surge) and monitoring the air pump(s) or powerhead(s).

Wet/dry filtration:This filtration method is the most widely used for saltwater reef aquariumstoday. Their popularity is due to its low maintenance and vast surface. Such a system is usuallyinstalled underneath the aquarium with a few being the hang-on-the-back variety. Water fromthe tank is sent to the filter by means of a pre-filter and siphon tube(s). The water is first passedthrough a mechanical filter of the pre-filter box where it can be easily removed and cleaned on aregular basis, then the water is allowed to trickle slowly over a series of shallow trays or spraybars containing barely moistened, biologically active filter medium (DLS, BioBalls, etc). >Because of abundance of oxygen, bacterial activity is significantly enhanced while the slowpassage of waste laden water over the filter media gives the bacteria maximal opportunity tocomplete the process of degradation. This treated water is then returned to the tank via a waterpump. Such a system gives a hobbyist very high water quality and can sustain quite a bit oforganisms for the tank size.

One advantage of having a wet/dry filtration system are that you can increase your stockinglevels over undergravel filters significantly. Another advantage is that you can incorporatemechanical as well as chemical filtration into such a system by using one of the available filtertrays that usually come with wet/dry units. A disadvantage to such a system is the reportedincrease of nitrates over time to your tank. This filters tank us a considerable amount of space and their design facilitates evaporation so you will have to add water periodically or have a watermake-up system attached to the filter sump. These filters are also quite expensive with theaverage price for a 55 gallon wet/dry setup costing well over $200.

Other types of bilogical filtrtation:Another popular type of biological filtration for reef hobbyistis live rock filtration. This type of filtration relies solely on the live rock to manage the nitrogencycle. With a sufficient amount of live rock, this is a more natural way of providing biologicalfiltration for the reef aquarium. There are also lots of micro-organisms that inhabit the live rockthat help to make such a biological filtration system effective. Plenum systems are also usedtoday and this will be discussed in an upcoming talk given by another guest.

In closing,The type of filtration system required really depends upon what you what to keep inyou aquarium. If you budget is tight, then the obvious choice is to opt for the undergravel filtration method. If you want to have high levels of fish in your tank, then the wet/dry filtrationmethod seems best. Live rock filtration systems seem to be where everyone is moving towardthese days. All filtration methods can and have been used successfully and with propermaintenance, each type of filtration method can be effective for you.

Thank you very much and I will try to answer any questions.

Question: About UG Filters. You are saying that they will work fine with a "reef" tank for longterm sucess?

<MDB> It is possible, but it requires a lot of TLC

<MDB> I do not use a substrate at all, just a bare bottom

Question: How does the carrying capacity of live rock compare to wet/dry. (asker) consideringchanging "FO" tank to Berlin method

Question: Any opinion on running a reverse flow UGF?

<MDB> Live Rock is a better method to use in my opinion. I do not have the numbers on thetotal surface area of live rock vs wet/dry

<MDB> Reverse flow UG's work just as good as normal flow

Question: Do you have any further information on the fact discovered/published about 5 monthsthat Nitrosomonas sp. and Nitrobacter sp. are not the main bacteria involved in the ammonia tonitrate cycle?

<MDB> I have not read that report.

<MDB> Can you direct me to it?

Question: What about the efficteveness of different methods of foam fractionation ?

<MDB> I am afraid I do not understand the question on foam fractionation. This is an essentialpart of any marine aquarium

<MDB> Foam fractionation or protein skimming is used in conjunction with bilogical filtration

<EricEE> I think the question referred to countercurrent (airstone) skimmer vs. venturi vs.downdraft and their respective roles in the nitrification cycle.

Q: you run your tanks with at bare bottom, no substrate. Reasons?

<MDB> I do not use a substrate in order to siphon all dirt, detritus, uneaton food, etc.. from thebottom of my tank. I used to have sand and did not have good results and since I removed thesand, all is well and my "sand sifters" are thriving without it

Q: does that mean you do not use cucumber, brittle stars, gobies etc that are used for sandsifting?

<MDB> I have two cucumbers, 4 gobies and my brittle stars got moved to my fish-only tankbecause they started eating the worms on my worm-rock