Filtration - the key to a successful aquarium

Aquatropics Aquarium Center offers a variety of aquarium filtration products designed to help maintain your aquatic environment. Though the choices seem endless and the styles enormous, your decision should be based upon three basic functions: Biological, Particulate and Absorptive. That is it.

Biological filtration is preemptive to maintain a stable environment. The introduction of fish, plants, and food into your aquarium begins a series of processes, collectively known as biological filtration. In biological filtration, naturally present bacteria safely convert wastes into less toxic compounds. (See Fig. 1)

Particulate filtration is designed to remove soluble waste, thus alleviating excessive demand on the biological filter.

Absorptive filtration, often referred to as chemical filtration, utilizes compounds to absorb liquid impurities from the aquatic environment. Several products are available for aid in maintaining and/or altering water composition. A commonly used product is activated carbon, which is effective in removing discoloration and aroma.

More than 50% of waste produced by fish is in the form of ammonia, the majority of which is secreted through the gills. The remainder of the waste, excreted as fecal matter, undergoes a process called mineralization. Mineralization occurs when heterotrophic bacteria consume fish waste, decaying plant matter and uneaten food, converting all three to ammonia.

As ammonia levels rise in the aquarium, another group of bacteria, Nitrosomonas, feed on the ammonia and convert it to nitrite. In Fig 2 , you can see that ammonia levels reach a peak and then decline, corresponding to the production of nitrite. Nitrite is consumed by the bacteria Nitrobacter and is converted to nitrate, the end product of the biological filtration process.


Nitrite conversion normally occurs within 4 - 6 weeks of setting up a new tank. Nitrate levels will continue to rise unless removed through regular partial water changes or utilized by live plants.

Heterotrophic bacteria, Nitrobacter and Nitrosomonas , are beneficial bacteria that break down fish waste into less toxic compounds. Disease-causing bacteria, such as Columnaris and Aeromonas , are also present. Poor water quality contributes to the rapid multiplication of harmful bacteria and depresses the immune system of fish, increasing the possibility of sickness or death. Testing your water regularly is therefore critical to your success as a hobbyist.


pH is an important factor in your aquarium. It is responsible for controlling many chemical balances, including the ratio of nontoxic ammonium (NH4+) to toxic ammonia (NH3), and between the nitrite ion (N02-) and nitrous acid (HNO2). The term pH stands for the power of hydrogen and is a measure of the amount of hydrogen ions present in your water. ( See Fig. 3)


The pH scale runs from 0.0 to 14.0. Values less than 7.0 are acidic, 7.0 is neutral, and values greater than 7.0 are basic. ( Base/basic has formerly been referred to as alkalinity. Alkalinity is actually a measure of the buffering capacity of water.)

The pH in a freshwater aquarium should be between 6.0 and 7.8. it is important to know the pH requirements of the species of fish being kept. ( See Fig. 4) Marine species should be kept at a pH of 8.2.


As stated earlier, the majority of fish wastes produced in your aquarium is converted to ammonia. Ammonia exists in two forms: as toxic ammonia (NH3), and as the nontoxic ammonium ion (NH4+). At any given time, the amount present in each form is primarily dependent on pH, and to a lesser extent, temperature. Ammonia is more toxic at a high pH and high temperature than at a low pH and low temperature.

All of the test procedures used in ammonia testing measure total ammonia, which is a combination of ammonia and ammonium. To properly determine the amount of toxic ammonia, you need to know the pH and temperature of your aquarium water. Utilizing the chart in Fig 5 , you can easily calculate how much toxic ammonia is present. For example: Upon testing your aquarium water you find that there is 2.5 ppm (mg/L) [parts per million or milligrams per liter], total ammonia, the temperature is 75 ° F and the pH is 8.0. From the chart, you find that the corresponding factor is 0.0502. By multiplying the amount of total ammonia by this factor, you are able to determine the amount of toxic ammonia present in your water.

2.5 ppm ammonia x 0.0502 = 0.1255 ppm toxic ammonia.


Calculated toxic ammonia levels should not rise above 0.05 ppm, as higher levels can stress the fish and possibly cause gill damage or fish death. Visual signs of ammonia toxicity may include fish gasping at the surface of the water, cloudy eyes, and frayed fins. If toxic ammonia levels above 0.05 ppm are present, it is advisable to do a partial water change with water at a pH of 7.0 and reduce feeding. Do not add any more fish until the ammonia levels have decreased.


Alkalinity, or pH stability, is the measure of the buffering capacity of water, which is the ability to maintain a constant pH over an extended period of time. Alkalinity can vary depending on the water source and the environment, fish that favor 80 ppm or greater, include Rift Lake Cichlids and brackish water fish. Areas where granite and sandstone are predominant, as well as rain forests, have low mineral concentrations and can have an alkalinity below 40 ppm. Fish that favor a low alkalinity environment, 80 ppm or below, include South American Cichlids such as Angel Fish and Discus. ( See Fig 4) Saltwater fish should be maintained at an alkalinity of 170 ppm or above.

Water with a high alkalinity will have a pH range of 7.4 to 8.4. The pH will be very stable, making it extremely difficult to lower. In the aquarium, alkalinity is gradually reduced naturally by the ongoing nitrogen cycle. Water with a low alkalinity, if not properly monitored, will allow the pH to drop to dangerously low levels in a short time. A pH of 6.2 and lower, will start to inhibit the nitrogen cycle, resulting in the increase of nontoxic ammonium. When this pH is raised rapidly, through additives or a water change, the nontoxic ammonium, will be dramatically converted to toxic ammonia causing stress and fish losses. It is important to test the pH and alkalinity on a routine basis because, if ammonia is present, any adjustment that greatly increases the pH and alkalinity will also increase toxic ammonia levels.


Nitrite (NO2-), is produced from ammonia by Nitrosomonas bacteria. Nitrite is toxic to fish because it interferes with the fishes ability to use oxygen. Nitrite is also pH dependent. If nitrite is present in your tank and the pH falls below 6.5, the nitrite will start to be converted to nitrous acid, which is also toxic to fish. If high nitrite levels are present, 1.0 ppm for freshwater and 0.5 ppm for saltwater, it is advisable to do a partial water change with water at pH of 7.0 and reduce feeding. Do not add any more fish until the nitrite levels have decreased.


Nitrate (NO3-) is the end product of the nitrogen cycle and is relatively nontoxic. Plants and algae use nitrates as a food source. In freshwater aquariums, it is recommended that nitrate levels be kept under 60 ppm (mg/L). Levels in excess of 100 ppm can be tolerated, but anything above this level should be avoided. Excessive nitrates will promote the growth of algae.

In marine aquariums, nitrate levels should not exceed 20 ppm. Reef tanks should have no detectable nitrate because it cannot be tolerated by marine invertebrates.

The most effective method of reducing nitrate levels is through partial water changes. The addition of live plants to your aquarium will also help to reduce high nitrate levels.


Total hardness is the sum of calcium hardness and magnesium hardness in water. Calcium (Ca2+) is an essential mineral, especially in salt water, for skeletal and shell development in fish, snails, and corals. It is also very important for fish breeding and egg development. Magnesium (Mg2+) found in trace amounts, is required by aquatic animals to maintain metabolism, and is a key element in chlorophyll, the green coloring in plants. Improper water hardness can cause stress and long-term problems and many prevent the hobbyist from keeping fish active and healthy. Certain species of fish thrive best within a range of hardness that resembles their natural environments.

The staff at Aquatropics will help you determine which filter or combination of filters are best suited for your aquarium based upon the needs of the environment. We welcome the opportunity to assist and simplify any filtration concerns to assure a low maintenance aquarium system for years of enjoyment.

You are welcome to shop our Aquatic Supply Online inventory for testing supplies and filtration needs.


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