What Is the Nitrogen Cycle?
Every living thing produces waste. Fish excrete ammonia through their gills and in their urine. Ammonia is highly toxic — it burns gill tissue, disrupts osmosis, and kills fish. In nature, rivers and lakes are vast enough that ammonia dilutes harmlessly. In a closed aquarium, it accumulates rapidly.
The nitrogen cycle is the process by which two groups of beneficial bacteria colonize your aquarium filter and convert this toxic ammonia into progressively less harmful compounds. Nitrosomonas bacteria convert ammonia (NH3) into nitrite (NO2) — still toxic, but less immediately so. Nitrospira bacteria convert nitrite into nitrate (NO3) — far less toxic and removable through water changes.
A "cycled" aquarium is one where these bacteria are established and numerous enough to process all waste the fish produce within 24 hours. Without cycling, ammonia and nitrite accumulate and kill fish over days to weeks in what is called "new tank syndrome."
Fishless Cycling: The Right Way
Fishless cycling establishes your bacterial colony before any fish enter the tank — no fish suffer through the process. It takes 4-6 weeks but requires only occasional testing and dose additions from you.
Step 1: Set up your tank completely (substrate, filter running, heater at target temperature, plants if desired, dechlorinated water). Do not add fish.
Step 2: Add an ammonia source. The cleanest method is pure ammonium chloride solution (Dr. Tim's Ammonium Chloride, available online). Dose to 2-4 ppm of ammonia as measured with your test kit. Alternatively, add a small piece of raw shrimp or fish food to decompose.
Step 3: Test every 2-3 days. Week 1-2: ammonia may drop slightly as first bacteria establish. Week 2-3: nitrite begins to rise — this is good news. Ammonia-eating bacteria are working. Week 3-4: nitrate appears, nitrite peaks and begins to fall. Both ammonia and nitrite converting bacteria are establishing.
Step 4: Cycle completion is when both ammonia and nitrite read 0 ppm within 24 hours of dosing 2 ppm ammonia. Test twice to confirm. At this point, do a 30% water change (to lower accumulated nitrate) and add your fish.
- ✦Use an API liquid test kit — test strips are notoriously inaccurate and will mislead you during cycling
- ✦Keep temperature at 26-28°C throughout cycling — beneficial bacteria grow faster in warm water
- ✦If nitrite is "off the charts" on your kit, dilute the sample 1:1 with distilled water and multiply the result by 2
Speed Up Cycling in Cambodia
Four methods to reduce cycling time from 4-6 weeks to 1-2 weeks: (1) Use established media from a healthy tank — a handful of gravel or a used sponge filter from a friend's cycled tank seeds billions of bacteria into your filter. Fastest and most reliable method. (2) Seachem Stability — bottled beneficial bacteria that establish within 7-14 days when dosed daily. Available in Phnom Penh aquarium shops. (3) Tetra SafeStart Plus — similar to Stability, pour entire bottle into filter area on setup day. (4) API Quick Start — another bottled bacteria option, typically 30-50% cycling time reduction.
Cambodia's warm climate actually helps cycling — bacteria multiply faster at 28°C than at 20°C. What takes 6 weeks in a European winter happens in 3-4 weeks in Phnom Penh without any acceleration.
Fish-In Cycling (Emergency Method)
Sometimes circumstances require adding fish to an uncycled tank — you receive fish as a gift, a previous tank breaks, or you did not know about cycling before buying. Fish-in cycling is possible but requires active management to protect the fish.
Fish-in cycling protocol: test ammonia and nitrite daily. Whenever ammonia rises above 0.25 ppm or nitrite above 0.25 ppm, perform a 25-30% water change with dechlorinated water at the same temperature. Dose Seachem Prime at every water change — it temporarily detoxifies ammonia and nitrite for 24-48 hours, giving bacteria time to process it.
Fish-in cycling takes 4-8 weeks with proper management. Stock lightly — only 1-2 small fish during cycling. Do not add more fish until cycle is confirmed complete. Feed minimally (once per day, tiny amount) to reduce ammonia production.
After the Cycle: Maintaining Bacteria
Beneficial bacteria live primarily in your filter media — sponge, ceramic rings, bio-balls. They die when cleaned in tap water (chlorine kills them), when medication is added to the tank (antibiotics and copper kill them), or when the filter is not running for more than a few hours.
To protect your cycle: always rinse filter media in old tank water, never tap water; avoid adding medication directly to the main tank when possible; use a hospital/quarantine tank for sick fish; if you must medicate the main tank, add Seachem Prime daily during treatment and test ammonia for 2 weeks afterward.