The Nitrogen Cycle and Sponge Filters

Understanding the nitrogen cycle should be the top priority of every aquarist. Proper management of the waste produced by an aquariums inhabitants is essential for the ongoing health and stability of the aquarium. All living things in the aquarium, including fish, invertebrates, fungi, and some bacteria release nitrogen waste in the form of ammonia, (ammonium in acidic water) which must then go through the various stages of transformation by bacteria called the nitrogen cycle in order to be non toxic.

Bacteria that are often called nitrifires, break down the ammonium and covert it to nitrite, (which is toxic at high levels) and then another form of bacteria coverts the nitrite to nitrate, (a less toxic form of nitrogen) Eventually nitrate is further broken down to harmless nitrogen gas that escapes through the water surface of the aquarium.

In well established and mature aquariums, this is an ongoing process. In a newly established aquarium it can take weeks or months for bacteria colonies to become established enough to handle the bio load and keep everything in check. Unstable conditions, overcrowding and over feeding all play a role in either prolonging the establishment of bacteria or overloading the bacteria’s ability to keep up. Even in established aquariums the nitrogen cycle may be interrupted if the bacteria colony is wiped out or over loaded.

Biological filtration provides a medium for bacteria to colonize on that is rich in oxygen and comes in contact with circulating water. Sponge filters are ideal for the nitrogen cycle for various reasons:

  1. The sponge material provides an enormous area for bacteria colonization. A two inch diameter by six inch long sponge is equivalent to a gravel bed one inch deep and three feet square.
  2. Portability: A sponge filter may be moved easily from one tank to another, and will quickly become colonized if placed in a well established tank first. It is easy to protect the bacteria from medicating the tank by simply removing the sponge filter and returning it once the medication treatment is done.
  3. Easy access with minimal disruption


A sponge filter’s efficiency is determined by various factors:

  1. Dimension of the body of the sponge. The more surface area that is exposed to the water, the more effective it will be.
  2. The amount of water passing through the sponge. Water laced with decaying organic material must come in contact with the sponge and the bacteria colony in order for nitrification to occur. Good circulation through out the tank and into the sponge filter is essential.
  3. Keep the sponge, particularly the surface area clean. If the pores of the sponge become blocked by particulate organic matter or algae, flow is restricted and the water cannot come in contact with the bacteria. If the sponge filter becomes overwhelmed by a large bio load of waste or dissolved organic compounds, regular water changes will relieve the burden.


Plants will also help to keep the system in check by absorbing nitrate and ammonium depending on which is more readily available to the plants. The rate of uptake by the plants of nitrogen is dependent on the growth rate and if conditions are favorable for plant growth, (namely light intensity and availability of nutrients other than nitrogen).

Sponge filters are one effective tool in properly managing the nitrogen cycle of any aquarium.