How to cycle an aquarium

Why Cycle an Aquarium?

• Fish, shrimp, and other aquatic animals produce ammonia (NH₃/NH₄⁺) as waste.

• In a new tank, there aren’t enough beneficial bacteria to convert ammonia into less toxic compounds.

• Over time, two groups of nitrifying bacteria will colonize surfaces (especially in your filter and substrate):
   1. Ammonia-oxidizing bacteria (AOB) convert ammonia → nitrite (NO₂⁻).
   2. Nitrite-oxidizing bacteria (NOB) convert nitrite → nitrate (NO₃⁻).

• Once ammonia and nitrite remain near zero and you see nitrate rising, the cycle is considered “established.”

• If you add animals too early, ammonia or nitrite spikes can stress or kill them (a phenomenon known as “New Tank Syndrome”). 

Methods of Cycling

There are multiple ways to cycle a tank. Here are the most common:

For shrimp, bettas, or sensitive community fish, fishless cycling or media transfer seeding are the safest choices.

Step‑by‑Step: Fishless Cycling (Recommended for Sensitive Livestock)

This method ensures minimal risk to your future inhabitants.

Step 0: Prepare Equipment and Materials

• Aquarium tank, substrate, filter, heater (if needed), decorations, etc.

• Dechlorinator / water conditioner (to neutralize chlorine / chloramine in tap water).

• A reliable test kit for ammonia, nitrite, and nitrate (daily or every-other-day testing is ideal).

• Pure ammonia (ammonium chloride) solution (without additives or scents) OR a source of organic ammonia (e.g. shrimp bits) if you choose. DrTim's Aquatics+2AquariumStoreDepot+2

• (Optional) A bacterial starter supplement or “filter starter” product. Some help, but results vary. forums.thinkfish.co.uk+3Wikipedia+3Aquacadabra+3

Step 1: Set Up the Tank (Without Livestock Yet)

1. Place substrate, decorations, and arrange your layout.

2. Install your filter, heater, and any other equipment.

3. Fill the tank with dechlorinated water (treat tap water to remove chlorine/chloramine).

4. Turn on all equipment (filter, heater, flow) and let it run continuously.

5. Optionally, add your bacterial starter (read instructions).

   • Also possible: take a small piece of filter media or sponge from an established, healthy tank and place it in your new filter or tank. This “seeding” can speed the process. Aquarium Co-Op Forum+3aquariumadvice.com+3Reef2Reef+3

6. Wait a day or two to let things settle (check for leaks, equipment function).

7. Optionally, raise the temperature slightly (within safe limits for your planned livestock) and maintain stable pH to encourage bacterial growth. Aquacadabra+3Fresh Water Systems+3forums.thinkfish.co.uk+3

Step 2: Introduce Ammonia (as “Food” for Bacteria)

You need a source of ammonia to feed the nitrifiers:

• Using pure ammonia (ammonium chloride):
   • Dose the tank to around 2–3 ppm (mg/L) of ammonia. (Do not exceed ~5 ppm).
   • Retest every 24–48 hours.

• Or using organic matter (e.g. decaying shrimp):
   • Cut small bits of shrimp and place them in the tank. They decompose and release ammonia.
   • But this method is less controlled. Aquacadabra+3DrTim's Aquatics+3AquariumStoreDepot+3

Record how much you dose (especially with pure ammonia) so you know your “daily ammonia addition.”
Start testing ammonia, nitrite, and nitrate every 1–2 days.

Step 3: Monitor and Wait for Bacterial Colonization

As the cycle progresses, you’ll see a typical pattern:

1. Rising ammonia (because only ammonia is present first).

2. After some days, nitrite begins to appear (ammonia is being converted).

3. Eventually, ammonia starts to drop as AOB consume it and convert to nitrite.

4. Then nitrite peaks, followed by a decline as NOB convert nitrite → nitrate.

5. Nitrate becomes measurable and rises steadily.

Continue dosing ammonia to maintain ~2–3 ppm while you wait for stable conversion.
Continue testing every 24–48 hours.

This process typically takes 4 to 8 weeks, depending on conditions (temperature, pH, seeding, etc.). aquariumadvice.com+4Fresh Water Systems+4Aquarium Co-Op+4

You’ll know the cycle is mature when ammonia = 0, nitrite = 0, and you are seeing nitrate consistently, even after dosing ammonia. You may repeat dosing or increase until your filter can handle more.

Step 4: Partial Water Change (Optional, When Cycle Completes)

• Once the cycle is stable, perform a 25–50% water change to reduce excess nitrate.

• Replace the water with dechlorinated water matching temperature and similar parameters (pH, GH).

• Make sure not to “clean” or discard the filter media excessively — you want to preserve as much beneficial bacteria as possible.

Step 5: Introduce Livestock Slowly

• Start with just one or two hardy animals (e.g. shrimp or a hardy fish) and observe for a few days.

• Monitor ammonia and nitrite. If they remain at zero, you can gradually add more.

• Do not add your full stocking all at once.

• Maintain routine testing and partial water changes to manage nitrates. Aquarium Co-Op+2Fresh Water Systems+2

Special Considerations for Shrimp, Bettas, and Community Fish

• Shrimp are especially sensitive to ammonia/nitrite spikes. It is critical that the tank is fully cycled (or nearly so) before introducing any shrimp.

• Bettas prefer stable water, gentle flow, and clean parameters. Introduce them after you confirm the cycle is stable.

• Community fish vary in sensitivity. Even “hardy” fish will suffer if ammonia or nitrite spike.

• When introducing multiple species, consider compatibility, bioload, and stocking density gradually.

• Always acclimate new livestock slowly (drip acclimation, temperature match, etc.).

Troubleshooting & Tips

• If ammonia or nitrite spike excessively, do a partial water change (20–30%) to dilute and protect future animals.

• Do not overfeed in early stages — excess food leads to higher ammonia and complicates the cycle.

• Avoid cleansing or replacing filter media during the cycling process (this removes beneficial bacteria).

• Use stable temperature and moderate pH to encourage bacterial growth.

• If cycling very slowly, consider seeding media from an established aquarium (if safe).

• Be patient — rushing the process almost always leads to losses.

• Use high-quality test kits (liquid or strips) and record your data to see trends over time.