Aquaponics ammonia, nitrite, and nitrate are the three nitrogen readings that tell you whether your fish are safe and your plants are fed. FAO’s aquaponics manual gives clear targets: ammonia (total ammonia nitrogen) and nitrite should be near zero in a mature system and are toxic above about 1 mg/L, with nitrite causing problems from as low as 0.25 mg/L. Nitrate is far less toxic; FAO recommends keeping it between 5 and 150 mg/L and exchanging some water when it climbs higher. High aquaponics ammonia or nitrite means the biofilter is too small or not working; low nitrate means too many plants or too few fish.
Key takeaway: Aim for aquaponics ammonia and nitrite at zero and nitrate between 5 and 150 mg/L. Test weekly once the system is stable.
| Reading | Target (FAO) | Problem level | What it means |
|---|---|---|---|
| Ammonia (TAN) | Near 0 | Above 1 mg/L; above 4 mg/L inhibits bacteria | Biofilter undersized or failing; overfeeding |
| Nitrite | 0 | Problems from 0.25 mg/L; toxic above 1 mg/L | Nitrite-oxidizing bacteria not keeping up |
| Nitrate | 5–150 mg/L | Below 5–10 mg/L (low) or above 150 mg/L (exchange water) | Low: more feed or fewer plants. High: more plants or water exchange |
How the Nitrogen Cycle Works in Aquaponics
Fish excrete ammonia, mostly through their gills, and uneaten feed and solid waste break down into more ammonia. FAO describes nitrification as a two-step bacterial process: ammonia-oxidizing bacteria convert ammonia into nitrite, then nitrite-oxidizing bacteria convert nitrite into nitrate. Plants then absorb nitrate as their main nitrogen source. In a balanced system, aquaponics ammonia and nitrite barely register on a test because bacteria process them as fast as fish produce them. See aquaponics cycling for how this colony is built.
Aquaponics Ammonia: Why pH and Temperature Matter
Aquaponics ammonia exists in two forms in water: ionized ammonium (NH4+), which is much less toxic, and un-ionized ammonia (NH3), which is highly toxic. Test kits measure both together as total ammonia nitrogen (TAN). FAO explains that in acidic water, ammonia binds with hydrogen ions to form ammonium, while at pH above 7 more stays in the toxic form, and warm water makes this worse. So the same TAN reading is more dangerous at pH 8 and 86°F than at pH 6.5 and 70°F.
FAO also notes aquaponics ammonia above about 4 mg/L inhibits and drastically reduces nitrifying bacteria. That can spiral: an overwhelmed biofilter loses bacteria, so ammonia rises even faster.
Signs of Aquaponics Ammonia Stress
- Red or inflamed gills.
- Red streaks on the body.
- Lethargy and gasping at the surface.
- Rubbing against the tank sides.
Nitrite: “Brown Blood Disease”
FAO says nitrite can harm fish at concentrations as low as 0.25 mg/L, and high levels can cause rapid deaths. Nitrite stops blood from carrying oxygen, turning it a chocolate-brown color, a condition called “brown blood disease.” Affected fish gasp at the surface even when oxygen levels are high. Nitrite spikes often appear during cycling and right after new fish are added.
Nitrate: The Plant Food
Nitrate is the goal of the biofilter and is far less toxic to fish. FAO notes fish can tolerate up to about 300 mg/L, and some up to 400 mg/L. But FAO also warns that levels above about 250 mg/L can harm plants, causing excessive leafy growth and hazardous nitrate accumulation in leaves. The recommended working range is 5–150 mg/L.
- Nitrate too low (below 5–10 mg/L for weeks): FAO suggests slowly increasing feed (never leaving uneaten feed), adding fish, or removing some plants.
- Nitrate rising above 150 mg/L: exchange some water and use the removed water to irrigate other crops, or add more plants.
- Nitrate stable but plants show deficiencies: FAO says calcium, potassium, and iron may need supplementing.
What to Do When Aquaponics Ammonia or Nitrite Spikes
- Stop or reduce feeding for a day or two; feed is the main source of aquaponics ammonia.
- Do a partial water exchange with dechlorinated water at the same temperature.
- Increase aeration to help fish and bacteria. See dissolved oxygen.
- Check pH. If it’s high, ammonia is more toxic. Avoid sudden pH swings; FAO says changes of more than 0.3 in 12–24 hours can harm fish.
- Look for causes: dead fish, rotting feed, a clogged filter, a recent cleaning that removed bacteria, a pump failure, or overstocking.
- Increase biofilter area if spikes keep happening. FAO says high ammonia or nitrite means the biofilter surface area should be increased.
How Often to Test
- During cycling: every 2–3 days, as FAO recommends.
- After adding fish: daily for a week or two.
- Stable system: FAO recommends testing nitrogen every week.
- Anytime fish act strangely: test right away.
Liquid test kits for freshwater aquariums are inexpensive and accurate enough for home systems. Keep reagents in date and follow the instructions carefully. A pH meter helps too; see pH and EC meters.
Reading the Numbers Together
| Ammonia | Nitrite | Nitrate | Likely situation |
|---|---|---|---|
| High | Low | Low | Early cycling or biofilter crash |
| Low | High | Low/rising | Mid-cycle; nitrite bacteria still establishing |
| Near 0 | 0 | Rising | Healthy, mature system |
| Near 0 | 0 | Very low | Too many plants or too little feed |
| Near 0 | 0 | Over 150 mg/L | Too few plants; exchange water |
For balancing fish and plants, see best fish for aquaponics.
Choosing and Using Test Kits
For home aquaponics, freshwater aquarium liquid test kits are the usual choice. A few habits make them more reliable:
- Test at the same time of day so results are comparable.
- Rinse test tubes with sample water first and use clean tubes for each test.
- Shake reagent bottles as directed; nitrate kits in particular often need vigorous shaking.
- Read colors in daylight or under white light, against a white background.
- Replace reagents by the expiration date.
- Keep a log so you can spot trends, not just single readings.
Remember that kits measure aquaponics ammonia as total ammonia nitrogen. Because FAO notes ammonia is more toxic at higher pH and temperature, always read your ammonia result alongside pH and water temperature.
FAQs: Aquaponics Ammonia, Nitrite and Nitrate
What is a safe aquaponics ammonia level?
Near zero in a mature system. FAO considers ammonia toxic above about 1 mg/L.
What is a safe nitrite level?
Zero. FAO says problems can begin at 0.25 mg/L.
What nitrate level is best?
FAO recommends 5–150 mg/L.
Why is my aquaponics ammonia high?
Overfeeding, overstocking, a dead fish, an undersized biofilter, or a new system that isn’t fully cycled.
Is high nitrate dangerous to fish?
Fish tolerate high nitrate, but levels above about 250 mg/L can harm plants, so exchange water above 150 mg/L.
Sources
- FAO — Small-scale aquaponic food production
- Photo credits (Wikimedia Commons, edited with titles): AQUAPONIE by Hopeballa (CC BY-SA 4.0); PH- und Leitfähigkeitsmessgeräte by Rotkaeppchen68 (CC0); Fish farming on Øksfjorden in Loppa, Troms og Finnmark, Norway, 2022 August by Ximonic (Simo Räsänen) (CC BY-SA 4.0); Water droplets on a late cotoneaster (Cotoneaster coriaceus) leaf, Jardim da Fundação Calouste Gulbenkian, Lisbon, Portugal julesvernex2 by Jules Verne Times Two (CC BY-SA 4.0); Solanum lycopersicum in aquaponics at the NC Greenhouse Nursery by Gogerr (CC BY-SA 4.0)