💦 Aquarium Water Parameters
Ideal pH, hardness, temperature, and nutrient levels for six fish habitat types. Enter your current readings for instant pass/fail status.
Select Fish Habitat
Check Your Readings
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The Nitrogen Cycle Explained
The nitrogen cycle is the most important concept in fishkeeping. Understanding it helps you maintain safe water and avoid "New Tank Syndrome" — the leading cause of fish death in new aquariums.
Fish Waste & Uneaten Food → Ammonia (NH₃)
Fish excrete ammonia directly through their gills and in their urine. Decaying organic matter (food, dead plant material) also releases ammonia. Ammonia is highly toxic — even 0.25 ppm can stress fish and damage gill tissue. In a new tank with no beneficial bacteria, ammonia spikes rapidly.
Ammonia → Nitrite (NO₂⁻) via Nitrosomonas bacteria
Beneficial bacteria (primarily Nitrosomonas species) colonise your filter media and convert ammonia to nitrite. Nitrite is also highly toxic — it prevents fish blood from carrying oxygen, causing "brown blood disease". This stage typically begins 1–2 weeks into cycling a new tank.
Nitrite → Nitrate (NO₃⁻) via Nitrospira bacteria
Nitrospira bacteria convert nitrite to nitrate. Nitrate is relatively harmless at low levels but accumulates over time. High nitrate (over 40 ppm in freshwater, 20 ppm in marine) causes long-term stress, reduced immune function, and algae blooms. Regular water changes export nitrate from the system.
Nitrate Removed via Water Changes (or Denitrification)
Regular partial water changes (25–30% weekly) are the primary way to control nitrate. In planted tanks, live plants absorb nitrate as fertiliser. Advanced systems use denitrification reactors or refugiums (marine) to convert nitrate back to harmless nitrogen gas. A fully cycled tank shows 0 ammonia, 0 nitrite, and low nitrate.
Toxicity Quick Reference
| Parameter | Safe | Caution | Danger |
|---|---|---|---|
| Ammonia (NH₃) | 0 ppm | 0.25 ppm | > 0.5 ppm |
| Nitrite (NO₂⁻) | 0 ppm | 0.25 ppm | > 0.5 ppm |
| Nitrate (NO₃⁻) FW | < 20 ppm | 20–40 ppm | > 40 ppm |
| Nitrate (NO₃⁻) Marine | < 5 ppm | 5–20 ppm | > 20 ppm |