The Nitrogen Cycle

The Nitrogen Cycle is the single most important concept in fishkeeping. It is the biological process that converts toxic fish waste into safer compounds.

If you do not understand this cycle, your fish will die. This is not an exaggeration; it is the #1 cause of failure for new hobbyists.

The Three Stages of Toxicity

Stage 1: Ammonia ($NH_3$)

• Source: Fish poop, uneaten food, decaying plant leaves.
• Toxicity: Extremely High. Even invisible amounts can burn fish gills and cause death.
• The Hero: Nitrosomonas bacteria consume Ammonia and convert it into...

Stage 2: Nitrite ($NO_2^-$)

• Source: The byproduct of Ammonia oxidation.
• Toxicity: High. It binds to hemoglobin in fish blood, preventing them from absorbing oxygen (Brown Blood Disease).
• The Hero: Nitrobacter / Nitrospira bacteria consume Nitrite and convert it into...

Stage 3: Nitrate ($NO_3^-$)

• Source: The byproduct of Nitrite oxidation.
• Toxicity: Low. Fish can tolerate moderate levels (up to 40ppm) without acute issues.
• Removal: Bacteria do not eat this (in aerobic tanks). YOU must remove it via:
  1. Water Changes: Physically removing old water and replacing it.
  2. Live Plants: Plants consume Nitrate as fertilizer.

How to "Cycle" a Tank

"Cycling" a tank means growing enough beneficial bacteria before adding fish.

The Fishless Cycle (Recommended)

1. Set Up: Fill tank, turn on filter. bacteria live in the filter media, not the water.
2. Dose Ammonia: Add pure bacterial starter or a pinch of fish food to decay.
3. Wait: Test water weekly. You will see Ammonia spike -> then drop. Then Nitrite spike -> then drop.
4. Finish Line: When you can dose Ammonia and have it drop to 0ppm (and Nitrite 0ppm) within 24 hours, leaving only Nitrates, your tank is cycled. This usually takes 4-6 weeks.

"New Tank Syndrome"

If you add fish to a sterile, uncycled tank, they will produce Ammonia. Since there are no bacteria to eat it, the levels rise rapidly, leading to stress, disease, and death within the first week. This is avoiding by cycling first!

Conclusion

Patience is key. You cannot see the Nitrogen Cycle, but it is the heartbeat of your ecosystem. Respect the biology, test your water, and your inhabitants will thrive.