The Fine Points of Filtration

Maintaining ideal water conditions in a fish tank is a constant challenge. Unlike the open natural environments from which fish originate, our tanks are closed systems that accumulate waste products. As these wastes accumulate, water conditions deteriorate. This is why filtration is so important for maintaining good water quality. Filtration takes potentially harmful compounds and makes them less harmful, either by removing them from the water or by transforming them within the tank into compounds that are less potentially dangerous.

There are many ways a hobbyist can provide filtration. Broadly speaking, there are three types of filtration: mechanical, chemical and biological. Mechanical filtration traps particulate waste in some sort of material so it can be physically removed from the tank. Chemical filtration uses specialized materials or devices to remove or convert dissolved materials. With biological filtration, natural processes break down potentially harmful compounds and reduce them to less harmful compounds. Each method attacks the problem of pollutant accumulation from a different direction, so all three methods play a useful role in maintaining water quality.

Mechanical filtration
Mechanical filtration is the hobbyist's first line of defense in maintaining a healthy tank. Uneaten food, along with waste products from the tank's inhabitants, can significantly degrade water quality if allowed to remain in the tank. At first, these materials are fairly benign, but they gradually decay. As they decay, they are transformed into a multitude of organic compounds, and these are what we need to be concerned with. If there is some sort of mechanical filtration system that traps uneaten food and solid waste, and if the filter media is cleaned or replaced on a regular basis, the proportion of wastes allowed to break down declines.

Mechanical filtration can be accomplished in many ways. Most self-contained power filters include a mechanical section consisting of coarse material that traps large particles. This material is typically replaceable, so it can be periodically discarded (with some filters, it can be washed and put back into service).

Canister filters use an internal or external pump to circulate water through the filter media. The bigger canisters have upward of 25 square feet of filtering material and can process as much as 1,000 gallons of water an hour. Depending on the biological load of the system, canister filters can often handle mechanical filtration tasks for tanks as large as 200 gallons.

The key to successful mechanical filtration is regular cleaning or replacement of the filtration media. The accumulated wastes will begin to deteriorate as soon as they are captured by the filter. The more frequently the filter is cleaned, the more wastes are removed from the tank. Ideally, media should be cleaned or replaced weekly.

Chemical filtration
Regardless of the effectiveness of mechanical filtration, some wastes will not be trapped. Often, very fine materials get through, so some particulate matter will remain in the water. In addition, because mechanical filters remove wastes from the water column, wastes lying on the bottom of the tank will not be removed. Hobbyists therefore need methods to process the wastes that remain in the aquarium. This is where chemical filtration can help. Chemical filtration removes or breaks down the dissolved materials that decaying wastes produce. This filtration can also directly or indirectly remove other contaminants, such as heavy metals.

The most common chemical media is activated carbon. Carbon is "activated" by creating microscopic fissures in the surfaces of carbon particles. Large organic compounds are trapped in these fissures and removed from the water. Activated carbon will not remove metabolic by-products, such as ammonia and nitrite. Because metals often bind to the large molecule compounds, they too can be removed with carbon. One popular use of carbon is removal of the yellow cast created by tannins.

Activated carbon is made from either wood or coal, and the size of the fissures is determined by the source material. Some sources create very fine fissures that do an excellent job filtering air particles but are less effective in filtering the larger organic molecules found in fish tanks.

Carbon's effectiveness also depends on the amount of particulate matter reaching it. Particulate matter can quickly clog the fissures on the surfaces of the particles, rendering them ineffective. Once the fissures are clogged, carbon becomes just a mechanical filter. For this reason, it's best to have some sort of mechanical filtration before the water reaches the carbon. This will delay clogging and extend the effectiveness of the carbon. Power filters that combine mechanical and chemical filtration always place the mechanical section ahead of the chemical one.

If water entering the carbon filter has first passed through a mechanical filter, a small amount of carbon will last quite some time in the average fish tank. For example, just a few ounces will keep a typical tank free of yellowing compounds for at least a month. If a hobbyist notices a yellow cast to the water, it is time to replace the carbon. New carbon filters typically have some carbon dust, so they should be rinsed before use.

Activated carbon is the most common chemical filtration media because it is readily available, relatively inexpensive and equally effective in both freshwater and saltwater systems. Some manufacturers combine carbon with ion exchange resins or phosphate removers. Resins are designed to remove ammonia and other nitrogen compounds. Phosphate removers can be helpful if a hobbyist's aquarium has chronically elevated phosphate levels.

There are other "chemical" filtration methods that work well in larger systems but which require greater attentiveness on the part of the hobbyist. Ultraviolet sterilizers reduce pathogens by exposing them to ultraviolet light. Sterilizers can greatly reduce water-borne bacteria and also "polish" water by reducing the yellow cast that can develop.

Another method is ozone, a highly reactive form of oxygen. Injecting ozone into the water improves water quality by oxidizing large organic compounds. Although ozone can be effective, particularly if used with other methods such as activated carbon, hobbyists should be very cautious with it. Large amounts of ozone can be hazardous to both aquarists and tank inhabitants. Too much ozone can "burn" delicate tissue such as fish gills. Excessive ozone can be particularly dangerous in saltwater tanks because it reacts with some elements of saltwater, such as boron, producing potentially toxic compounds. Begin with very modest levels somewhat below the recommendations for your tank size and observe the tank's inhabitants. If any of the tank's inhabitants seem to be negatively affected, then ozone dosage should be reduced. It also helps to monitor ORP (Oxidation-Reduction Potential) levels to make sure they increase modestly and gradually. At proper levels, one should not be able to smell ozone around the tank. The odor of ozone is slightly sweet, like the air after a thunderstorm.

Another chemical filtration method used in freshwater ponds and saltwater tanks is foam fractionation or protein skimming. Foam fractionation capitalizes on the fact that organic molecules (such as proteins) are attracted to air bubbles. Fractionation circulates water through a stream of bubbles. As the bubbles rise, they attract organic molecules. The resultant foam is then "skimmed" by collecting and removing the foam. Some hobbyists inject ozone into a protein skimmer because it collects small organic molecules more efficiently than large molecules, and ozone can break large organic molecules into smaller ones.

While all these chemical methods work individually, they tend to work much better when combined. Activated carbon can be combined with an ultraviolet sterilizer, and, as noted, ozone can be injected into a protein skimmer. The synergism of combining methods improves the effectiveness of water processing and thus does a better job of improving water quality.

Biological filtration
Biological filtration refers to a natural process in which organic waste products are broken down by bacteria in a succession of steps referred to as the nitrogen cycle. As organic waste products initially break down, they produce highly toxic ammonia. In addition, tank organisms excrete ammonia, which further increases its concentration in the tank. Nitrifying bacteria feed on the ammonia, producing nitrite as a by-product. Although this eliminates ammonia from the tank, nitrite is nearly as toxic as ammonia. Fortunately, other bacteria feed on nitrite, producing nitrate as a by-product. These bacteria thrive on all tank surfaces, so in a sense, every tank is a biological filter with nitrifying bacteria performing their important job.

Tanks with large surface areas and light biological loads may need nothing more than bacteria to control ammonia. However, if an aquarium has measurable ammonia, it means there is not enough surface area for bacteria populations to reach the necessary densities to completely eliminate ammonia and nitrite. A dedicated biological filter is necessary.

So-called "trickle" filters and fluidized bed filters increase the effective surface area over which bacteria can grow, therefore accommodating a higher bioload. Relatively modest-sized trickle filters and fluidized beds can convert large amounts of nitrite into nitrate.

Trickle filters consist of an open container filled with plastic media through which water flows. The media are designed to offer a large surface area for a given volume, so as oxygenated water flows through the media, the large surface area promotes the growth of high densities of aerobic bacteria.

Fluidized beds use a different approach to achieve the same goal. Sand particles are suspended and recirculated in a volume of water so that each sand grain is bathed with oxygen-enriched water. The large surface area of sand grains promotes the rapid growth of bacteria. The challenge for hobbyists is how to remove the nitrate that accumulates as a result of this natural biological process.

Nitrate is not nearly as toxic as ammonia or nitrite. Some aquarists believe that concerns about nitrate are excessive and that periodic water changes can maintain reasonable nitrate levels. Fish seem to acclimate to elevated nitrate levels if these gradually increase. However, newly added fish that have not been gradually acclimated to these increasing nitrate levels may suffer nitrate shock.

Invertebrates are far more sensitive to nitrate than fish, so maintaining low nitrate levels is more critical in systems with these animals. This is why reef tank hobbyists are more concerned about nitrate than most other hobbyists.

As with the other stages of the nitrogen cycle, bacteria play a central role in controlling nitrate in a tank. Bacteria can consume nitrate and release nitrogen gas. The bacteria that consume nitrate, however, require quite a different environment from the bacteria that produce nitrate.

Nitrifying bacteria are aerobic bacteria. They grow in the presence of oxygen (in an oxic environment). In contrast, denitrifying bacteria are anaerobic bacteria, which grow in low and oxygen-free environments (suboxic and anoxic areas). If a hobbyist can create anoxic areas in a tank, bacteria can provide a natural means of reducing. The challenge is creating such an environment. Several decades ago, reef hobbyists discovered that carbonate-based rocks ("live rock") harbor anaerobic bacteria. They found that with sufficient live rock, nitrates can be virtually eliminated, so live rock is commonly used in marine aquariums, both in reef systems and fish-only tanks.

Filtration is a critical component in creating and maintaining a healthy environment for fish and invertebrates. Mechanical, chemical and biological filtration benefit all tanks. A small effort in setting up and maintaining good filtration systems will pay large dividends in creating a healthy aquatic environment.