Master the nitrogen cycle! Learn how beneficial bacteria keep your fish alive, and how to cycle your tank fishless.
wasserwerte · 7 Min. Lesezeit · nitrogen cycle, cycling, bacteria, water quality, beginner
The Nitrogen Cycle Explained
The nitrogen cycle is the biological process that keeps your fish alive. Understanding it is not optional — it's the foundation of successful fishkeeping.
The Problem: Fish Produce Toxic Waste
Fish breathe, eat, and excrete waste. Their primary waste product is ammonia (NH₃/NH₄⁺), which is extremely toxic even at 0.25ppm.
Ammonia burns gills, damages organs, and kills fish quickly.
The Solution: Beneficial Bacteria
Your tank needs two types of beneficial bacteria:
Nitrosomonas — converts ammonia → nitrite
Nitrospira — converts nitrite → nitrate
These bacteria colonize your filter media, substrate, and surfaces.
The Nitrogen Cycle in 3 Steps
Step 1: Ammonia Production
Fish waste, uneaten food, decaying plants
Toxic level: >0.25ppm
Step 2: Nitrite Conversion
Nitrosomonas bacteria convert ammonia to nitrite (NO₂⁻)
Also toxic: >0.25ppm, causes "brown blood disease"
Step 3: Nitrate Conversion
Nitrospira bacteria convert nitrite to nitrate (NO₃⁻)
Much less toxic: safe up to 20-40ppm
Removed through water changes and plant uptake
Cycling Your Tank (Fishless Method)
Never add fish to an uncycled tank!
Materials Needed:
Ammonia source (pure ammonia or fish food)
API Master Test Kit
Patience (4-8 weeks)
Day-by-Day Process:
Week 1-2:
Add ammonia to reach 2-4ppm
Test daily
Ammonia starts dropping, nitrite appears
Week 3-4:
Continue adding ammonia to feed bacteria
Nitrite spikes (can reach 5+ ppm)
Nitrate begins appearing
Week 5-6:
Ammonia and nitrite both drop to 0 within 24 hours
Nitrate accumulates
When ammonia and nitrite stay at 0 for 7 days: You're cycled!
Final Step Before Fish:
Large water change (50-70%) to lower nitrate
Test again: Ammonia 0, Nitrite 0, Nitrate <20ppm
Now safe to add fish slowly!
Cycling with Fish (Not Recommended)
Fish-in cycling exposes fish to toxic ammonia/nitrite — it's stressful and often deadly.
If you must cycle with fish:
Use extremely hardy fish (Danios, White Cloud Minnows)
Stock very lightly (2-3 small fish max)
Test daily, water changes daily if ammonia/nitrite >0
Do not feed heavily (less food = less waste)
Seachem Prime can temporarily detoxify ammonia/nitrite during emergencies.
Maintaining Your Cycle
Once established, never crash your beneficial bacteria:
Do NOT:
❌ Replace all filter media at once
❌ Clean filter media in tap water (chlorine kills bacteria)
❌ Use antibiotics carelessly (some kill beneficial bacteria)
❌ Let filter run dry
DO:
✅ Rinse filter media in old tank water
✅ Replace filter media gradually (50% at a time)
✅ Keep filter running 24/7
✅ Maintain stable temperature (bacteria slow in cold water)
Crash Recovery
If your cycle crashes (after medication, power outage, etc.):
Immediately stop feeding
Daily 50% water changes until bacteria rebuild
Dose Seachem Prime to detoxify ammonia
Test daily
Consider bottled bacteria (Tetra SafeStart, Fritz TurboStart)
Recovery typically takes 1-2 weeks.
Signs of an Uncycled Tank
Fish gasping at surface
Red, inflamed gills
Lethargy, clamped fins
Ammonia/nitrite test positive
Fish deaths within days/weeks
Conclusion
The nitrogen cycle is not optional. Every successful tank depends on thriving beneficial bacteria.
Cycle fishless, test religiously, and never rush. Your fish's lives depend on it.