Saltwater Filtration 101, How it Differs from Freshwater

It seems that the most common questions asked by people who are thinking about setting up their first saltwater tank are always about filtration. What new equipment will I need? Can I use any of my freshwater filters? My freshwater filter is rated for a 125 gallon tank, why can't I use it on a saltwater tank?
These questions dominate the internet forums and by far the most complicated questions to answer quickly. For this reason, I decided today that I would spend some time and try to write what I hope becomes a useful article. We'll see how this goes.

First and foremost, we have to ask this question. What are we trying to accomplish with filtration? This seems simple enough, but there are many different types of filters on the market today and they each accomplish different tasks. We also need to discuss what these filters do by accident... what are the "side effects" so to speak. Keep this in mind as we discuss filtration concepts and how they differ in marine aquariums.

Lets start by explaining what we are trying to accomplish with filtration in a marine aquarium. In a marine aquarium our goal is stability. We want to create an environment which is as stable as possible, with as little interference from the fishkeeper as possible. To do this, first, want to keep Nitrates near zero or zero. Next we want stable alkalinity and calcium levels. Third, we seek to minimize the introduction of Phosphates into the aquarium. Finally, we need to ensure ammonia and nitrite do not accumulate, and at the same time accomplish all of the goals above. We can't just do one of these and not the others. All of these are of equal importance for the success of our systems.

Now that we understand our goals, lets start looking at freshwater equipment and its application to the marine hobby. Most freshwater systems today have hang on filters, equipped with biological, mechanical, and chemical filtration. It is assumed that you understand these 3 types of filtration.

Lets start by discussing biological filters. Biological filters come in many types. There are sponge filters, trickle filters, wet dry filters, ceramic rings and bioballs, and even undergravel filters. Each of these filters is designed to break down ammonia into nitrite, and nitrite into nitrate. Nitrate accumulates and is only removed or controlled with water changes. In a marine aquarium this presents an obvious problem, because we are trying to REDUCE the level of Nitrate to zero. It is not practical to intentionally use a filter which increases nitrate.

Further, many people do not realize that the process of nitrification also depletes carbonates from the buffer system, lowering alkalinity. Specifically, 8.64 mg of alkalinity in the form ofHCO3- are consumed per mg of ammonia-nitrogen oxidised. (ref: http://www.fishdoc.co.uk/filtration/nitrification.htm) Lets make sure you understood this last part. Again, the process of biological filtration lowers alkalinity. So, to summarize, the use of biological filtration causes an increase in nitrates and a lowering of alkalinity. Both of these concepts conflict with the goals we have, so the use of a biological filter in a marine aquarium is not ideal for long term success.

{update: this was recently questions on the threads, so I will provide a 2nd reference on this. You will have to copy paste the entire link, which is to a google scholar article Adapted from North American 2006 Veterinary Conference. Here is the link:
http://74.125.155.132/scholar?q=cache:E9_fh5P8vNUJ:scholar.google.com/+nitrogen+cycle+carbonate+depletion+aquarium&hl=en &as_sdt=4000000 }

Next, lets consider chemical filtration. Chemical filtration is generally seen in aquariums with the use of activate carbon. The carbon absorbs acids directly from the water. In freshwater aquariums, far to many hobbyists ignore this filtration option, presumably due to costs and misunderstanding the benefits. However, in a marine aquarium the benefits of activated carbon or other organic acid absorbing resins are something to consider.

The benefits of activated carbon in a marine aquarium are 3 fold. First, carbon absorbs organic acids, which reduces the amount of waste that eventually breaks down into nitrate. Second, these organic compounds tint the water, reducing light penetration, which is critical in marine aquariums housing corals. Finally, removing these acids helps reduce the usage of carbonates from the buffer system, which helps stabilize alkalinity. I use activated carbon in my marine aquariums, and I think that you should also.

The benefits of other resign medias vary, but the most commonly used is a phosphate absorbing resign. Phosphates are of a great deal of importance in marine systems, because they are a leading cause of algae and cynobacteria, both of which can cause havoc in a marine system. Phosphates are introduced in many ways, so having a phosphate sponge resign is a good form of added insurance. I use a phosphate sponge in my reef, and I think you should also.

The final form of filtration on a freshwater system is mechanical filtration. Mechanical filtration exists in every freshwater aquarium in one form or another. Generally, water is forced through filter pads, and the pads catch free floating particles, removing them from view. It is important to note that these particulates are NOT removed from the system. The water still flows through the filter pads, coming in contact with the particles. This is of concern in a marine system, because the flow of water across these trapped particles causes a biological process to occur which results in a release of phosphates. As mentioned above, phosphates are bad. So, the bottom line with mechanical filtration is this. If you want to use filter floss or other types of filter pads, be sure to rinse them DAILY to remove all seen and UNSEEN particles which have become trapped in the floss. I personally do not use filter floss of any type in my marine tanks, and I think you should follow my lead.

So, lets take a look at where we are now. We have eliminated biological filters and mechanical filters as ideal filtration methods for a marine aquarium. We have also decided that chemical filters have a place, but these certainly are not going to get the job done alone. So, where do we turn? The answer lies in forms of filtration that are not seen on freshwater systems. In fact, these concepts do not exist in freshwater aquariums. These concepts are Protein Skimming, Live Rock, and Live Sand.

It is getting late and this thread is running long, so I think i will finish this up tomorrow in another post.

The purpose of this article is not to explain the installation of all types of filtration options, or is it to compare the different brands and make recommendations. My goal is to help you understand what the benefits and drawbacks are of each filtration option, so that you can make an educated decision on what type of filtration you want to use on your new marine aquarium. (My real goal is to encourage you not to use a biological filter on a marine aquarium, but I will keep that secret to myself, and hope you come to this realization on your own.)

Lets start by discussing Protein Skimming. In a nutshell, a Protein Skimmer removes organic wastes directly from the water, before they have the opportunity to break down into ammonia. Let me say that again, because it is critical. Protein Skimmers REMOVE the waste directly from the water. They do not break down waste into some other form. Inside the skimmer column, waste bonds to the bubbles, and is removed into the collection cup of the skimmer. This provides an amazing advantage over the other filtration options discussed above, because there is no end product (Nitrate) in this method. A properly sized protein skimmer is extremely effective at removing organic acids directly from the water, allowing you to run an aquarium which has an extremely slow and very small buildup of nitrates.

There are other benefits to skimming as well. One often ignored benefit is perhaps the most important. By removing the organic acids directly from the water, the skimmer is removing waste without the process of nitrification having occurred, as seen in biological filtration. This helps to maintain a stable alkalinity, which in turn helps to maintain calcium levels. Proper calcium and alkalinity levels help to encourage coraline algae growth. Coraline algae discourages the growth of problematic algae, such as hair algae and cynobacterias.

Another huge benefit of protein skimming is saved medical bills, for both you and your fish. It is a commonly known fact that carrying large buckets of water is very painful for your back. I can assure you that the amount of water changes you do with a good quality skimmer will be very minimal, compared to other filtration options. You see, i've just saved you a few trips to the chiropractor. Also, the overall stability of your system will be much improved, with nitrate and alkalinity fluctuations being minimal. This improves the health of your livestock, saving you endless trips to the fish vet. This is amazing, I just solved the health care crisis in less than 30 seconds. 8)

The only thing left to discuss is the removal of ammonia, and of the VERY slow increase in Nitrates that occurs on systems with a Protein Skimmer as the filtration of choice. This removal of ammonia and nitrate is where live rock and live sand come in to play.

First, it is important to know that live rock and live sand do not have to be "live". You can use large amounts of dry rock and dry aragonite sand, seeded with small amounts of actual live rock and sand. Within a few shorts months all of the rock and sand will be live. This saves you a ton of money and makes the marine hobby affordable for many people who otherwise would not have the budget to purchase live rock. I personally used nearly 90% dry rock in my 180 gallon FOWLR aquarium. I do not think it is appropriate to discuss vendors or product choices in an article such as this, but I am more than happy to discuss specific recommendations if you post a question on the forum , or you can read for yourself how my systems was created here http://www.tropicalfishkeeping.com/s...f-build-21979/. The point is, live rock should not be an expense that makes this hobby unaffordable.

Live rock and live sand are natural biological processors of waste. They break down ammonia into nitrite, nitrite into nitrate, and nitrate into nitrogen gas. It is important that you understand the difference between a biological filter and the natural processing of waste that occurs within live rock & sand. Biological filters produce nitrate as an end product. Live rock and sand produce nitrogen gas as the end product. The nitrogen gas leaves the system naturally, and is harmless to your livestock.

This process of converting nitrate to nitrogen gas is only possible in systems with live rock and sand*, and is greatly enhanced with the use of a deep sand bed. (Sand bed depths of 4'' - 6'' are ideal.) This is possible due to denitrifying bacteria which grow deep inside the pores of the live rock and grow in the sand bed. Denitrifying bacteria do not grown on man-made biological filters, such as sponge filters, biowheels, and undergravel filters. It is the growth of these bacteria that make the entire system extremely stable. In fact, most people using a Protein Skimmer, live rock, and deep sand bed, have Nitrate reading at or near ZERO. The Protein Skimmer REMOVES the bulk of the organic waste. Any waste left behind is processed by the live rock and sand, into nitrogen gas.

*{disclosure: It is possible to buy denitrification filters, but these are expensive in our hobbyand unnecessary. In some rare cases, studies have also shown that trickle filters set up correctly can harvest denitrifying bacteria, but the negative drawback of detritus accumulation and phosphate production makes these filters impractical for the marine hobby.}

Suffice to say that my recommendation for filtration in a marine aquarium is Protein Skimming, Live Rock, and a sand bed of 4''-6'' depth. That being said, there are some systems which are not going to utilize live rock, I understand this. Sometimes the pure aesthetics of the situation require minimal amounts of live rock, and modification need to be made to these systems. In these rare situations, I suggest using an oversized skimmer, in combination with a trickle filter (bioballs). The key is this: ALL WATER must flow through the protein skimmer BEFORE it comes in contact with the trickle media. This allows the skimmer to remove the organic wastes directly, and dramatically limits the amount of nitrification that takes places within the trickle filter. This type of setup is very valuable on systems with very high bioloads, or little to no access to live rock.
I hope this article helps to explain some of these filtration topics to a beginner in the marine hobby. I want to arm you with enough information to ask intelligent questions on the forum. I also hope that you read this prior to your trip to the LFS, or at least before wasting money on equipment purchases or upgrades that do not apply to the marine hobby.

Thanks for reading & Happy Fishkeeping!