What is fishless cycling?

Fishless cycling is the process of establishing the nitrogen cycle in a new aquarium without any fish present. You add a small amount of ammonia to feed the beneficial bacteria that will eventually keep your tank safe for livestock. Because no animals are involved, you can let the process run at its own pace without the pressure of protecting fish from toxic spikes.

The nitrogen cycle works in two stages: Nitrosomonas bacteria convert ammonia (NH₃) to nitrite (NO₂⁻), then Nitrospira bacteria convert nitrite to nitrate (NO₃⁻). Nitrate is far less toxic and is removed by regular water changes. Once your filter can process a dose of ammonia to zero within 24 hours, your tank is cycled and ready for fish.

When to choose fishless cycling

• You are setting up a new tank and have not bought fish yet
• You can wait 4–6 weeks before adding livestock
• You want the lowest-risk approach with no chance of harming fish during cycling
• You do not have access to a healthy established tank for seeding media

What you’ll need

• A liquid ammonia and nitrite test kit (API Master Kit covers ammonia, nitrite, nitrate, and pH)
• A nitrogen source: pure ammonia (clear, unscented, no surfactants — shake test: if it foams, reject it) or fish food as an alternative
• Dechlorinator (e.g. Seachem Prime, API Stress Coat)
• A running filter with filter media installed
• A heater keeping water at 22–26 °C (72–79 °F) — warmth accelerates bacterial growth

Step by step

Step 1 — Fill tank and add dechlorinator

Fill the tank with tap water and dose dechlorinator according to the bottle instructions. Chlorine and chloramine kill beneficial bacteria, so this step is non-negotiable. Run your filter and heater from day one. The warmer the water, the faster bacteria colonise.

Hint: Use a quality dechlorinator and follow the dosage on the bottle.

Step 2 — Add ammonia source to reach 2–4 ppm

Add pure ammonia or fish food to bring ammonia to 2–4 ppm. Test immediately after dosing. Add small amounts, test, and repeat until you hit the target. Do not exceed 4–5 ppm — very high ammonia inhibits the bacteria you are trying to grow.

Hint: Use pure ammonia (no surfactants) or fish food to provide the nitrogen source.

Step 3 — Wait for ammonia spike, then nitrite rise

Test daily. Within 1–2 weeks you should see ammonia start to drop and nitrite begin to rise. This is the first stage of the cycle — Nitrosomonas are colonising your filter. Keep ammonia between 1–4 ppm by topping up when it drops below 1 ppm. Do not let it hit zero before nitrite clears or the colony may crash.

Hint: Test daily. Nitrosomonas bacteria will start converting ammonia to nitrite — usually takes 1–2 weeks.

Step 4 — Nitrite drops to 0 and nitrate rises

Nitrospira bacteria now colonise and convert nitrite to nitrate. This phase typically takes another 1–2 weeks. You will see nitrate climbing while nitrite falls. Maintain ammonia as in Step 3. Some cycles stall here — patience is the only solution.

Hint: Nitrospira bacteria convert nitrite to nitrate. This phase can take another 1–2 weeks.

Step 5 — Confirm cycle complete — do a final water change

Add 2 ppm ammonia. Retest after 24 hours: if both ammonia and nitrite read 0 and nitrate has risen, the cycle is complete. Do a 50% water change to reduce nitrate before adding fish. Add fish slowly — start with a light stocking level to avoid overwhelming the colony.

Hint: Add 2 ppm ammonia. After 24h both ammonia and nitrite should read 0. Do a 50% water change before adding fish.

Common mistakes

Using cloudy or surfactant-containing ammonia. Ammonia sold for household cleaning often contains detergent. Shake the bottle — if bubbles persist, reject it. Buy unscented, clear ammonia from a hardware store or dedicated aquarium cycling product.

Letting ammonia drop to zero before nitrite clears. Beneficial bacteria starve quickly. If you leave for a few days without topping up, the colony can crash and you may need to restart. Set a reminder to test and dose every 2–3 days.

Adding fish before fully verifying the cycle. A single good reading is not enough. Dose 2 ppm, wait 24 hours, and confirm both ammonia and nitrite read 0 on two consecutive days before introducing any livestock.

How long to expect

Phase	What’s happening	Typical duration
Days 1–7	Ammonia builds, no bacteria yet	1 week
Days 7–21	Ammonia drops, nitrite rises	1–2 weeks
Days 21–42	Nitrite drops, nitrate rises	1–3 weeks
Day 42+	Cycle confirmed, ready for fish	—

Temperature, tank size, and media type all affect speed. Cold water (below 20 °C) significantly slows the process. A seeded sponge from a friend’s established tank can cut weeks off the timeline.

AquaKeepers has a built-in fishless cycling guide that walks you through each of these steps — it tracks your progress, logs your parameter readings, and tells you when your tank is ready for fish.