Animal-World > Aquarium Tropical Fish > Cycling Your Aquarium

Aquarium Nitrogen Cycle
Start Cycling Aquarium Guide

Picture of a cycled aquarium   Cycling your aquarium is the first thing, and the most important thing you must do to provide a healthy and stable home for your fish!

   Aquarium maintenance is easy if you have an aquarium nitrogen cycle going on. The aquarium cycle is more commonly known as the nitrification process and is the biological filter for your tank. Aquarium filtration systems work to keep your water pristine and provide a safe home for your fish, and the biological filter is perhaps the most important filtration system to have.

There are three methods of cycling an aquarium covered in this article.

   In the table below there are Three Methods for Cycling a New Tank, but first let's look at why you want to have a biological filter and cycled your tank. Let's explore what the aquarium nitrogen cycle is and how it works. Then we'll cover some important things to watch out for to keep it working!

To set- up and prepare your aquarium before cycling it - and to learn about adding fish - see:
Quick Guide to Setting Up a Freshwater Aquarium
Quick Guide to Setting Up a Marine Aquarium


Why cycle your aquarium:   The important reason for cycling an aquarium is because of deadly ammonia present in the aquarium. Ammonia kills fish and ALL fish tanks produce ammonia Ammonia enters the aquarium through either fish waste, uneaten foods, and/or detritus that start decomposing. Ammonia can be removed through chemical filtration, but that method requires an ongoing testing and maintenance chore, usually weekly.

   A cycled aquarium maintains itself through a natural biological process. Beginning the aquarium nitrogen cycle and continuing the cycle for the entire life of the tank are important to keeping fish alive and healthy. The cycle starts when ammonia becomes present in the tank. Ironically, though ammonia will kill your fish, it is essential for the nitrification cycle. It is the 'food' that feeds the beneficial bacteria, allowing the bacteria to live and thrive, which then provides a balance environment so your fish will thrive.

Simple! How the nitrogen cycle works:    The nitrification cycle is actually pretty simple. It can seem hard because of a couple terms that are not familiar in our everyday language. But basically this is a simple three-step process.

  1. Step One The first thing that happens when you put fish in the tank is ammonia is produced. This is from the fish waste or excess foods that are decomposing.
  2. Step Two As the amount of ammonia starts to increase, a bacteria forms called nitrosomonas. This bacteria begins to convert the ammonia into nitrite. As the ammonia is converted to nitrite, the amount of ammonia will begin to drop and now the nitrites will begin to rise. Soon your ammonia test will show no more ammonia in your tank. This usually happens within the first week and a half of a normal cycle. Nitrite is also very toxic to fish, though not as hazardous as ammonia.
  3. Step Three As the nitrite levels increase, another bacteria forms, called nitrobacter. This second bacteria begins to convert the nitrite into nitrate. As it is converted to nitrate, the amount of nitrite will drop and the nitrates will begin to rise. Soon your nitrite test will show no more nitrite in your tank. This happens between three to six weeks in a normal cycle. Nitrate is harmless to fish but is one of the nutrients that plants and algae need and is the final product produced in the nitrification cycle. One of the reasons that water changes are recommended is to keep nitrates at lower levels.

   The time it takes to cycle an aquarium can be sped up by 'seeding' the aquarium with commercially prepared bacteria. Then the entire cycle will only take between 1 - 2 weeks. This seed bacteria is available in either a freeze dried or liquid form. You can also add bacteria rich media from an established aquarium, like some of the gravel. The seeding should be done after ammonia is starting to form in the newly setup aquarium. You must wait until there is ammonia or the seeded bacteria will starve.

What about the nitrates produced:    Nitrate is the end product of the nitrogen cycle. The simplest way to remove nitrates is through regular water changes. For the most part nitrate is not harmful to fish unless in enormous quantities and for prolonged periods of time, and even then it is only some types of fish are at risk. You will most likely not run into this problem in a regular home aquarium.
   Nitrates can actually be beneficial if you have live plants, as it is a nutrient for them. Tanks without live plants to utilize this nutrient however, can get excess algae growth. This holds true for both freshwater and saltwater fish only aquariums, but not for the reef aquarium. Nitrates are usually strictly controlled in the reef aquarium because they can cause undesireable algae growth.

What is needed for cycling success:    Besides fish that are producing ammonia, there are two important things the nitrogen cycle depends on:

  1. Needs a place for beneficial bacteria to live and grow: When you set up your aquarium you will be using a filter of some sort which will provide a home for this bacteria. The most common filters are undergravel filters, external filters, or internal filters. Each of these has a media with a lot of surface area for the bacteria to live and grow on. On the undergravel filter the media is the gravel itself, other filters use some sort of synthetic filter media, such as foam or filter pads.
       The most important thing to think about when choosing the media is that it provides a lot of 'surface' area for the bacteria colony to grow on. Sponges are laced with holes so a lot of surface area is created inside the entire sponge, pads are similar.
  2. Needs oxygen to survive:    The higher the oxygen content of the water, the healthier the bacteria will be. To have oxygen in your water, the water needs to be flowing. Where the water is exposed to the air, usually on the surface, an exchange happens. Here other molecules in the water rise to the surface and are exchanged with the oxygen molecules. Then your filters pump moves the water through the aquarium, and the newly oxygenated water flows over the bacteria.

Tips to keeping your tank cycled:    Once your tank is cycled there are some things to be aware of so that you don't loose your beneficial bacteria. When the bacteria is removed or dies, the ammonia levels begin to rise and you can quickly lose your fish.

  • Make sure your pump (filter) is always working. Water that is not moving becomes stagnant. If your pump quits or the water stops flowing for some other reason, it is estimated that it takes about 6 hours for the bacteria to die from lack of oxygen.
  • If your filter media becomes exposed to air and dries out your bacteria will die.
  • When you remove the sponge or pad media from your filter to clean it, you can easily wash the beneficial bacteria off of it. It is best if you have two sponges or pads. This way you can swap between the two each time you do maintenance, cleaning one and leaving the other. Next time you do maintenance clean the opposite one. It takes about 1 - 2 weeks for a cleaned pad to re-colonize when there is a healthy colony still in the aquarium.
  • When you remove the sponge or pad media from your filter to replace it, you loose all the bacteria growing on it. Again it is best to have another sponge or pad, and only replace one at a time.

   If you loose your beneficial bacteria or it dies, you will have to re-cycle your aquarium again to grow a new batch of bacteria!

Three Methods for Cycling a New Tank
Cycling the Aquarium - Aquarium Nitrogen Cycle

Described here are three different methods to cycle your aquarium.

   You will need to monitor the ammonia and nitrite levels (using test kits) throughout the cycle period, until they are both zero (or very close to zero). When they are both zero, the aquarium is cycled and is safe for fish.

Method 1 - Cycling with fish

  This first method is the classic 'traditional' method used for many years. It cycles the aquarium using inexpensive hardy fish. Hardy fish to cycle an aquarium with include Danios, Platies, and even goldfish.

1. Traditional Method:

  1. Have the temperature stabilized at 74° to 80° F(26° - 28° C).
  2. Place 1 hardy, inexpensive fish for each 2-3 gallons of aquarium water. Inexpensive fish include danios, platys, barbs, mollies, etc. These fish will provide the initial ammonia to get the biological filter started (see Biological filtration above). This should take about thirty days to six weeks.
  3. This can be stressful to the fish, especially if you add large numbers of fish. Fewer fish will be less stress as changing water parameters go slower and they have a chance to adjust.
  4. After about six weeks, when the aquarium has "cycled", you can add additional fish (see nitrification cycle under Biological filtration above).

Method 2 and 3 - Fishless cycling:
Cycling an aquarium without fish

   These next two methods cycle the aquarium without the use of fish. One is with the use of ammonia, and the other is with the use of fish food.
These points apply to both of the non-fish methods:
  • Higher temperatures of 86° - 95° F ( 30° - 35° C) can be used for optimum bacteria when cycling without fish, but you must stabilize the aquarium slowly back to lower temperatures before you add fish.
  • You can speed and enhance the nitrification process by introducing a starter culture of bacteria. One way to do this is by is seeding the aquarium with some gravel or filter media with existing bacteria from an established aquarium. Another way is by adding commercial preparations of nitrifying bacteria, there are several different brands available at pet stores.

2. Cycling using ammonia:

  1. Introduce pure ammonia to cycle the aquarium. You can buy unscented ammonia with no additives from a supermarket or a bottle of ammonium chloride.
  2. Add ammonia from a dropper, 3 - 5 drops per 10 gallons of water per day to get and maintain a reading of 5 ppm.
  3. Initially there will be no nitrites. Monitor nitrites daily and continue the daily ammonia dose until you get a nitrite reading. At this point you can reduce the daily amount of ammonia to 2 - 3 drops per 10 gallons. Continue this until both the ammonia test and the nitrite test reads 0 ppm.
  4. This method can take as little as three weeks or up to six weeks to complete the nitrification cycle, but adding a starter culture as described above can speed the time up considerably.
  5. When the cycle is complete reduce the temperature slowly back to 74° to 80° F(26° - 28° C). Reducing it quickly can stress the bacteria.
  6. Do a major water change, about 90%, and add activated carbon to remove any possible additives which might have been in the ammonia.

3. Cycle using fish food:

  1. Simply feed the tank with a fish food, presumable daily to keep an ongoing decomposing process. As the food decays it will to produce ammonia and get the biological filter started.
  2. This method takes about the same amount of time as the fish method above.
  3. The main drawback to this method is that it is difficult to get a large enough initial bacteria colony. So when you introduce the fish, they may add a larger ammonia load than the colony can handle. Consequently you may get some additional ammonia and then nitrite spikes, though they should be less dramatic and shorter lived than the initial cycling spikes.
  4. Another drawback is that the decaying food, besides producing ammonia, can add other by-products such as phosphates.


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