How to Start the Nitrogen Cycle in your Tank

Establishing a consistent and reliable nitrogen cycle on your tank is critical for the health of your fish. Before you purchase any fish, your cycle should be healthy.

Getting the Nitrogen Cycle Started

When you first get your tank started, you should put your substrate on the bottom, fill the tank with water, and start your pump with your filter. Assuming you used tap water, there will be a small amount of chlorine and chloramine in your tank. You can either add dechlorinator or leave the top off of your tank for 2-3 days to let the chlorine and chloramine evaporate.

By Aleš Tošovský [Public domain]
By Aleš Tošovský [Public domain]

Fishless Cycling (Strongly Recommended)

After the chlorine and chloramine is gone, you need to add ammonia to the tank.  There are 3 common sources for this ammonia:

  • 100% pure ammonia (recommended) – be sure to check for no fragrances, surfactants, etc.  If the ingredients say anything other than ammonia, keep looking
  • fish food
  • a chunk of raw shrimp or fish (not recommended in tanks tanks less than 30 gallons)

You are aiming for an ammonia level of 2-4 ppm (parts per million).  This is typically about 2-3 drops of pure ammonia pure 5 gallons of water or 0.75 ml per 5 gallons. With fish food, just drop a small pinch into your aquarium in the morning and at night for the length of the cycle.  It will slowly break down over the next few days and produce ammonia.

Leave the top of your tank off open to the air and keep your filter running.  Your filter plays a large part in establishing the healthy bacteria colonies.

After 3-5 days, you should begin doing daily water tests. Over the next week or so, bacteria in the air will fall into the water and those that eat ammonia will start to establish themselves (this can be sped up – more on this later). When you start to see the ammonia levels fall, add more pure ammonia to get back to 2-4 ppm.  If you are using fish food, keep up with the daily morning and night “feedings”. If you went with shrimp, you shouldn’t have to do anything.

At this point, you should start to see your nitrite levels begin to rise. You should continue to add your ammonia source  to “feed” the initial bacteria. Much like the bacteria with the ammonia, the same process as with the ammonia will begin to happen with the nitrites. Bacteria that eat nitrites will establish themselves in your tank. These bacteria will then excrete nitrates. Fish can tolerate much higher levels of nitrates than ammonia and nitrites.

At this point, you can slowly start to add fish. We recommend adding 10-20% of your final stock to the tank at this point. You should start with the most hardy fish first. A week later, test your water and make sure your ammonia and nitrites are zero. At this point you’ll do your first water change. From here, you should add 25% of your stock each week until you are fully stocked.

How to Start the Cycle with Fish (Not Recommended)

Once the chlorine is out of your tank, you should buy 2-3 of the most hardy fish from your planned stock and put them in the tank (see our fish conditioning guide here). You should begin doing daily water tests the following day. You should start to see ammonia building up in your tank. Then much like above, the ammonia is eaten by bacteria and nitrites are created. Then the nitrites are eaten and the result is nitrates.

We don’t recommend this method.  It puts a lot of stress on the fish.  The stress makes them more susceptible to disease and illness.  If you end up with a sick fish, the disease can be transferred to the other fish in your tank and possibly infect future fish that are added.

By Ilmari Karonen [Public domain]
By Ilmari Karonen [Public domain]

Stocking your Tank to Prevent “Mini-Cycles”

When your tank has just finished its first cycle and is ready for fish, its in a very delicate state. Adding too many fish at once can shock the system and force the aquarium into what are called “mini-cycles”. This is the result of the change in volume in ammonia that “feeds” the cycle. You’ve been feeding ammonia to the system and now that you have fish providing that ammonia, the volume is different.

The best way to prevent this is to cycle your tank and slowly add fish. Never add more than 25% of the final stock at a time and wait at least a week between additions. During that week, you should be keeping an eye on your water parameters.

How to Jump Start the Nitrogen Cycle

Normally, you would let bacteria in the environment naturally fall into your tank and their numbers would slowly build up. But you can jump start this process. You can take some substrate from an already established tank and add it to your tank. This will make the number of starting bacteria many times higher which will get things going much faster. If you don’t have a friend or relative that has a tank, you can check with your local fish store and they will normally be more than happy to give you a small amount of substrate to get you going. We would recommend avoiding some of the larger fish chains (you know who I’m talking about) as their tanks usually use the same water for all of tanks (they have a big pump system that constantly recycles the water) and any disease from one fish can easily infect all of their water. Bringing substrate or water from that system can be detrimental to your fish.

Brief Overview

  • Get your tank running by adding water and substrate, and get your filter pumping.
  • Get the chlorine and chloramine out either by adding dechlorinator or waiting a few days
  • Get some ammonia in your tank – either by adding ammonia, fish food, or raw shrimp or adding a few hardy fish
  • Test your water every couple days
  • When the ammonia levels begin to drop, add more ammonia (your nitrites will start going up at this point)
  • Continue “feeding” your tank ammonia
  • When your nitrite levels start going down, your nitrate levels should start going up
  • Keep adding ammonia until you are ready to stock your tank
  • Add a few of your most hardy fish to your tank
  • Keep an eye on your water chemistry – if the ammonia and nitrites are zero you’re good to keep adding fish
  • When your nitrates start getting above 20, prepare to do your first water change
  • Once you are comfortable that your tank is cycling properly, you can settle into once a week water parameters testing and water changes.

Quick Tips

  • Using pure ammonia as the source of ammonia is strongly recommended.  It gives you 100% control over your ammonia levels.
  • Can’t find pure ammonia at your local fish store? Try a hardware store or even a grocery store.
  • Crank up the temperature into the low 80s.  Bacteria reproduce quicker in this temperature range.
  • If you are using pure ammonia: When you start testing your water, check your ammonia levels first.  If that hasn’t changed since yesterday, then neither have your nitrites/nitrates.
  • “Instant Cycling” products are very hit or miss, even within the same product line. Use cautiously.
  • One of the best ways to quick start your cycle is by soaking a sponge in the water of an established tank for about a week.  This will give you the largest starting colony sizes for the required bacteria.
  • Larger tanks take longer to cycle than smaller tanks.  Bacteria can only reproduce so fast.
  • Cloudy water will very likely clear up once your tank’s cycle is established.