Nitrogen cycling happens in soil and water all around the earth. Aquaponics cycling is the natural process of bacteria oxidizing ammonia into nirates.


Cycling Process

The cycling process starts when ammonia is present in your aquaponic system. Ammonia (chemical formula NH3) is a compound made of nitrogen and hydrogen. Fish produce ammonia in the form of feces, urine, but mostly from respiration in the gills. You can also add pure ammonia in liquid or powder form. Just as our own waste is toxic to us, ammonia is toxic to fish and can kill them, and is not available as a nutrient for uptake by plants. The cycling process converts ammonia to a less toxic form of nitrogen  (nitrate) that plants can readily digest.

Ammonia attracts nitrosomonas, the first of the two types of nitrifying bacteria that will populate the surfaces of your system. The nitrosomonas bacteria convert the ammonia into nitrites (NO2). This is the first step in the aquaponics cycling process. Unfortunately, nitrites are even more toxic than ammonia! But the cycling process does not stop there. Nitrites attract the bacteria species nitrospiraNitrospira converts the nitrites into nitrates. Nitrates are generally harmless to the fish and an essential nutrient for your plants.

Cycling Startup

Cycling starts when first setting up or restarting an aquaponic system. The cycling process generally takes from 4-6 weeks. The time frame is dependent on the water temperature (ideally 75° – 80° F or 24° to 26° C). Water temperatures outside this range will take longer to cycle since the bacteria will be slower to eat and reproduce. Once cycling is complete, water temperatures should be between 70° to 75° F, 21° to 24° C). This is a temperature range that is good for both fish and plants.

You can speed up the cycling process by introducing nitrifying bacteria instead of waiting for them to appear on their own. Add a bacteria supplement to the media surface or water, so they can populate your aquaponic system faster.

The Aquaponic cycling process
Nitrification Cycle Chart


Fish Cycling

  1. Stat fish cycling by adding fish to the water. The fish will provide the ammonia source. Do not feed the fish for the first 24 hours.  Only lightly feed for the first several days.
  2. Add a bacteria starter or let the bacteria populate naturally.
  3. Perform a water test every day to check ammonia, nitrite, and nitrate levels. Ammonia should remain below 3.0 ppm. Nitrites should remain below 1.0 ppm. Nitrates will increase over time.
  4. Exchange 1/3 of the tank water if ammonia gets above 3.0ppm, or nitrites above 1.0ppm. Fish can suffer and may die if levels get higher than these.

Fishless Cycling

Cycling with fish can be stressful (on both you and the fish). An alternative is fishless cycling.

  1. Add a bacteria starter or let the bacteria arrive naturally.
  2. Add a supply of ammonia (powder or liquid) until levels are at 4.0 ppm.
  3. Perform water tests to detect ammonia, nitrite and nitrate levels.
  4. Continue to add ammonia to keep the ammonia level around 4.o ppm.
  5. Cycling is complete when ammonia and nitrite levels drop below 0.5 ppm with 24 hours. The presence of nitrates indicates cycling is taking place.
  6. Stop dosing the system with ammonia and add your fish into the system.  Do not feed for the first 24 hours.

Common Cycling Mistake

A common mistake is to add bacteria starter and ammonia only in the beginning. When a water test shows 0 – 0.5ppm ammonia, it is assumed the system is cycled. Really this means its time to add more ammonia. The ammonia needs to be available constantly for the bacteria to thrive. Another mistake is to add ammonia and have fish. Only one source of ammonia is necessary, and fish produce ammonia on their own. Adding ammonia supplement with fish in the tank is bad for their health.

Cycling is complete when a water test detects nitrate levels increasing. At the same time, ammonia and nitrite concentrations are consistently below .5 ppm or less. Your system will be fully cycled making the water safer for the fish and nitrogen available for the plants. Once the nitrification cycle starts, it will continue until the source of ammonia is removed or temperatures rise or fall dramatically.

You are a bacteria farmer. It is just as important to keep your bacteria happy as your fish and plants. Nitrifying bacteria are essential to the health of your aquaponic system. Nitrification happens all over the world. This is just one of the ways that your aquaponic system mimics a natural ecosystem.