Ammonia
NH3 (un-ionized) / NH4+ (ionized) • Inorganic nitrogenous compound
Ammonia (NH3/NH4+) is the most toxic compound in the aquarium nitrogen cycle, produced by fish waste and decaying organic matter, and safe only at a reading of 0 ppm.
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Important Checklist
- Test parameter
- Compare safe range
- Identify source
- Adjust slowly
- Retest
Troubleshooting Guide
1 Ammonia spike in a new tank
Expected during cycling. Perform daily 25-50% water changes and dose a detoxifying conditioner (e.g. Seachem Prime) until the biological filter establishes and readings reach 0 ppm.
2 Ammonia spike in an established, cycled tank
Indicates a filter crash or bacterial die-off. Check for recent filter media cleaning in tap water, antibiotic dosing, a dead/decaying animal, or a sudden increase in bioload. Perform an immediate large water change and dose a conditioner while the colony recovers.
3 Persistent low-level ammonia despite water changes
Test tap water directly for ammonia/chloramine, as some municipal water supplies use chloramine (which breaks into ammonia + chlorine). Switch to a conditioner rated to neutralize chloramine, not just chlorine.
Quick Answer
Ammonia (NH3/NH4+) is fish waste's first breakdown product and the most toxic compound in an aquarium. A cycled, healthy tank should always read 0 ppm; any positive reading calls for an immediate water change and a detoxifying conditioner while the biological filter catches up.
Safety Profile
- Safe Range
- 0 ppm-0 ppm ppm (Total Ammonia Nitrogen)
- Toxicity Level
- High (as free NH3)
- Toxicity Note
- Un-ionized ammonia (NH3) is toxic to fish and invertebrates even at trace concentrations, damaging gill tissue and impairing oxygen uptake. Toxicity rises sharply with higher pH and warmer temperature, since both shift the ammonia/ammonium equilibrium toward the more dangerous NH3 form.
Properties & Reactions
| Detection Method | Liquid test kits or test strips measuring Total Ammonia Nitrogen (TAN); pH must be read alongside the result to judge actual NH3 risk. |
| Common Sources | Fish gill excretion, Uneaten food decay, Decaying plant matter, Dead livestock/snails, Tap water chloramine breakdown |
| Converts To | Nitrite (chem-nitrite), oxidized by Nitrosomonas bacteria as the first stage of the nitrogen cycle |
| Influenced By | pH, Temperature, Bioload, Filtration capacity |
Related Compounds
Ammonia (NH3) vs. Ammonium (NH4+)
| Property | Ammonia (NH3) | Ammonium (NH4+) |
|---|---|---|
| Toxicity to fish | Highly toxic, even at trace levels | Comparatively harmless |
| Favored by | High pH, warm temperature | Low pH, cooler temperature |
| Detected by standard test kits | Combined as Total Ammonia Nitrogen (TAN) | Combined as Total Ammonia Nitrogen (TAN) |
| Safe aquarium reading | 0 ppm | Presence tolerated if pH keeps it as NH4+ |
Frequently Asked Questions
What is the difference between Ammonia (NH3) and Ammonium (NH4+)?
How quickly does an ammonia spike kill fish, and what are the signs?
Does Seachem Prime or API Ammo-Lock actually remove ammonia?
Why do I still have ammonia in an established, cycled tank?
Can live plants absorb ammonia?
Glossary of Terms
- Total Ammonia Nitrogen (TAN)
- The combined measurement of both un-ionized ammonia (NH3) and ammonium (NH4+) in water, as reported by standard aquarium test kits. Toxicity depends on the pH and temperature of the water.
- Un-ionized Ammonia (NH3)
- The toxic form of ammonia. Produced directly by fish waste and uneaten food. Even trace concentrations (above 0.02 ppm) damage gill tissue and suppress the immune system.
- Ammonium (NH4+)
- The ionized, comparatively harmless form of ammonia. Dominates in acidic (low pH) conditions. Standard test kits measure both NH3 and NH4+ as a combined TAN figure.
- Nitrogen Cycle
- The biological process by which beneficial bacteria convert toxic ammonia → nitrite → nitrate. A fully cycled tank has established Nitrosomonas and Nitrobacter colonies that process waste continuously.
- Nitrosomonas
- Aerobic bacteria responsible for the first stage of the nitrogen cycle — oxidizing ammonia (NH3) into nitrite (NO2−). They colonize filter media, gravel, and live rock surfaces.
- Biological Filtration
- The use of beneficial bacterial colonies housed in filter media or substrate to break down toxic nitrogenous waste. The primary long-term defence against ammonia and nitrite spikes.
Related Content
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Nitrite
Nitrite (NO2-) is the second, often more persistent, toxic stage of the aquarium nitrogen cycle, causing 'brown blood disease' by blocking oxygen transport in fish.
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pH
pH measures how acidic or alkaline aquarium water is on a 0-14 scale; stability and its effect on ammonia toxicity matter more than hitting one 'ideal' number.