How To Cycle A Tank: Understanding The Nitrogen Cycle

When it comes to any type of animal residing in a water filled tank or aquarium, it is important to understand the function that the nitrogen cycle plays in your tank environment. While actually a very simple process to understand, ignorance of it can lead to your pets swimming around in steadily toxifying water. On the flipside, a properly cycled tank stays cleaner, less smelly, and self-eliminates harmful waste compounds. Whether you own turtles, fish, or any other aquatic animal, learning how to cycle a tank is practical knowledge that will save you time and effort.

What Is The Nitrogen Cycle?

To put it simply, a nitrogen cycle is the presence of an active biological filter consisting of beneficial bacteria that break down nitrogenous aquatic animal wastes. Essentially, when your turtles poop or leave food uneaten, these things rot and break down. As they break down, ammonia is released into the water.

Ammonia is extremely toxic to marine life. Turtles are not as susceptible as fish and other marine animals since they spend time out of the water, but over time the ammonia will have a negative impact on their health.

Having a healthy nitrogen cycle means that you have established colonies of bacteria that break the ammonia down into nitrite, and then from nitrite into nitrate. Nitrate is significantly less toxic than ammonia and nitrate levels can be managed with just partial water changes. A proper nitrogen cycle allows you to gets away with doing only a 25% change of tank water per week, whereas an uncycled tank can develop dangerous levels of ammonia quickly and require 100% water changes to keep things in check. Also unlike ammonia, nitrate is removed by plants in your tank, which use it as food.


Quickstart Nitrogen Cycle

If you’re desperate to do a quick cycling of your tank, there are options for pre-bottled bacterial blends. However, these have pretty mixed reviews. Some people swear by them and some say they don’t work, but we would be remiss if we didn’t lay out your options. Two popular brands of bacterial starter are:


How To Cycle A Tank Naturally

An important thing to note is that natural nitrogen cycle establishment takes time, as much as 4 to 12 weeks. This may seem like a long time, but remember that your red eared slider can potentially live for 20 or 30 years. In comparison to their total lifespan, this is a relatively short amount of prep time that will have a drasticly positive impact on their health and longevity. Like a newborn baby, you want to be properly prepared when you bring home your first slider.

If you have the time and ability, it is best to at least try and start a natural nitrogen cycle, it’s really not difficult at all.

Tools you’ll need to start a natural nitrogen cycle:

NOTE: If you want to better understand the 3 chemicals we will be monitoring during the cycling process, take a quick peek at the Water Chemistry page.

The Process:

Step 1

  • In a separate container, take the water that you plan to use and add your water conditioner according to the instructions on the bottle. This will remove chlorine and chlorinates, which are present in most tap water and which will disrupt or ruin your nitrogen cycle. As an turtle owner this is a ritual you should get used to, as all water that comes into contact with your turtle’s tank filter or that you use for water changes need to be treated in advance for this reason.

Step 2

  • Add your treated water into the tank, and let the filter run for 24-48 hours to make sure everything is working.

Step 3

  • Use your testing kit to determine the ammonia, pH, nitrite and nitrate levels of your water so you have a set of base measurements.

Step 4

  • Following the instruction on the bottle, add ammonia to your tank water until you have achieved a concentration of of 3-4 ppm (parts per million).

Step 5

  • Wait 2 or 3 days and then measure the ammonia level, continue doing this at regular intervals until you notice the ammonia levels dropping. Around days 9-12, your ammonium should be dropping and you might be able to measure some nitrite.

Step 6

  • When your ammonia levels have dropped to 1ppm, add more ammonium to bring it back up to 3-4ppm. Somewhere between days 14 and 20 you should notice your nitrite start to spike. This means your first set of bacteria are taking hold and breaking down the ammonia, so you’re halfway there.

Step 7

  • At this point, you can just add small amounts of ammonia, as you want to avoid the nitrite levels rising over 5ppm, as this will poison the bacteria that will convert it into nitrate. Once these bacteria appear and you do start to see the nitrite decrease, it will drop very fast.

Step 8

  • Once the nitrite levels drop you should be getting measurable amounts of nitrate. At this point, all the bacteria that need to be present have established themselves. When you can add a full ammonia dose of 4ppm and by the next day have a reading of 0ppm ammonia, 0ppm nitrite, and a measureable amount of nitrate then you tank is fully cycled.


Cycle Bump:

Once you have a cycle established, you will be doing water testing every now and then to monitor the quality of your slider’s tank water. You may encounter what is known as a “cycle bump,” which is a small spike in ammonia or nitrite. This can occur if for some reason the bacteria colony is diminished or new ammonia sources are added to the tank. These will usually resolve themselves in a week or so as the bacteria colony adjusts itself to the new tank conditions.


Keep Reading: Red Eared Slider Care Sheet

  1. Tank Size and Setup
  2. Filter and Water Temperature
  3. Lighting/Heat Requirements
  4. The Nitrogen Cycle – Current Page
  5. Tank Cleaning and Water Chemistry
  6. Diet and Nutrition
  7. Illnesses and Injury
  8. Additional Reading and Resources