How an aquarium
‘cycles’
Call it cycling, nitrification, biological cycle, startup cycle, break-in cycle, or the nitrogen cycle. No matter what name you use, every newly set up aquarium goes through a process of establishing beneficial bacterial colonies. Older aquariums also go through periods during which the bacterial colonies fluctuate. Failure to understand this process is the largest contributing factor to the loss of fish. Learning what it is, and how to deal with critical periods during the nitrogen cycle, will greatly increase your chances of successful fish keeping.
The Waste Problem
Unlike nature, an aquarium is a closed environment. All the wastes excreted from the fish, uneaten food, and decaying plants, stay inside the tank. If nothing eliminated those wastes, your beautiful aquarium would turn into a cesspool in no time at all.Actually, for a short period of time, a new aquarium does become a toxic cesspool. The water may look clear, but don't be fooled, it's loaded with toxins. Sounds awful, doesn't it? Fortunately bacteria that are capable of converting wastes to safer by-products, begin growing in the tank as soon as fish are added. Unfortunately there aren't enough bacteria to eliminate all the toxins immediately, so for a period of several weeks to a month or more, your fish are at risk. However, you need not lose them. Armed with an understanding of how the nitrogen cycle works and knowing the proper steps to take, you can sail through the break-in cycle with very few problems.
Stages of the Nitrogen Cycle
There are three stages of the nitrogen cycle, each of which presents different challenges.
Initial stage: The cycle begins when fish are introduced to the aquarium. Their feces, urine, as well as any uneaten food, are quickly broken down into either ionized or unionized ammonia. The ionized form, Ammonium (NH4), is present if the pH is below 7, and is not toxic to fish. The unionized form, Ammonia (NH3), is is present if the pH is 7 or above, and is highly toxic to fish. Any amount of unionized Ammonia (NH3) is dangerous, however once the levels reach 2 ppm, the fish are in grave danger. Ammonia usually begins rising by the third day after introducing fish.
Second stage: During this stage Nitrosomonas bacteria oxidize the ammonia, thus eliminating it. However, the by-product of ammonia oxidation is nitrite, which is also highly toxic to fish. Nitrites levels as low as low as 1 mg/l can be lethal to some fish. Nitrite usually begins rising by the end of the first week after introducing fish.
Third stage: In the last stage of the cycle, Nitrobacter bacteria convert the nitrites into nitrates. Nitrates are not highly toxic to fish in low to moderate levels. Routine partial water changes will keep the nitrate levels within the safe range. Established tanks should be tested for nitrates every few months to ensure that levels are not becoming extremely high.
Second stage: During this stage Nitrosomonas bacteria oxidize the ammonia, thus eliminating it. However, the by-product of ammonia oxidation is nitrite, which is also highly toxic to fish. Nitrites levels as low as low as 1 mg/l can be lethal to some fish. Nitrite usually begins rising by the end of the first week after introducing fish.
Third stage: In the last stage of the cycle, Nitrobacter bacteria convert the nitrites into nitrates. Nitrates are not highly toxic to fish in low to moderate levels. Routine partial water changes will keep the nitrate levels within the safe range. Established tanks should be tested for nitrates every few months to ensure that levels are not becoming extremely high.
Now that you know what is happening, what should you do? Simple steps such as testing and changing the water will help you manage the nitrogen cycle without losing your fish.
How To Avoid Disaster
What To Do
The key for success is testing the water for ammonia and nitrites, and taking action quickly when problems occur. To aid in tracking the status of your aquarium, links to charts for logging your tests can be found under the charts section of this page. Each chart shows the danger zones and offers steps to reduce toxins before they result in loss of your fish.
Test for ammonia:
Test for ammonia:
Begin testing on day three after adding the fish, and continue every day until the ammonia begins to drop. After it begins to fall, continue testing every other day until the ammonia reaches zero. Using the chart provided, plot the ammonia levels. Should ammonia reach the danger zone, take steps as shown on the chart. If at any time fish show signs of distress, such as rapid breathing (gilling), clamped fins, erratic swimming, or hanging at the surface for air, take immediate action to lower the ammonia level. Chemicals such as Ammo-Lock will quickly neutralize toxic ammonia.
Test for nitrites:
Begin testing one week after adding the fish. Continue testing every second or third day, until it reaches zero. Using the chart provided, plot the nitrite levels and take steps as shown on the chart if nitrite reaches the danger zone. If at any time fish show signs of distress, such as rapid breathing or hanging near the surface seemingly gasping for air, test for nitrite. If levels are elevated perform an immediate 25-50% water change and test daily until levels drop.
What Not To Do
Don't add more fish - wait until the cycle is completed.
Don't change the filter media - the beneficial bacteria are growing there. Don't disturb them until they have become well established.
Don't overfeed the fish - when in doubt underfeed your fish. Remember that anything going into the tank will produce wastes one way or another.
Don't try to alter the pH - the beneficial bacteria can be affected by changes in pH. Unless there is a serious problem with the pH, leave it alone during the startup cycle process.
Don't add more fish - wait until the cycle is completed.
Don't change the filter media - the beneficial bacteria are growing there. Don't disturb them until they have become well established.
Don't overfeed the fish - when in doubt underfeed your fish. Remember that anything going into the tank will produce wastes one way or another.
Don't try to alter the pH - the beneficial bacteria can be affected by changes in pH. Unless there is a serious problem with the pH, leave it alone during the startup cycle process.
