How do I cycle my tank with pure ammonia (fishless cycling)?

Short answer: Set the tank up fully, dose aquarium-grade ammonium chloride to about 2–4 ppm, and test every 2–3 days. When nitrite appears, keep ammonia near 2 ppm. You're done when a fresh ~2 ppm dose is processed to 0 ammonia and 0 nitrite within 24 hours. On dosed ammonia alone this takes about 3–6 weeks; add bottled bacteria to cut it to 1–2 weeks.

The details — the steps

1. Set up completely first. Rock, sand, salt water at ~1.025 sg, heater at 75–78°F, and circulation pumps running. The cycle needs a stable environment.
2. Dose ammonia to 2–4 ppm. Use only reagent-grade or aquarium-specific ammonium chloride (e.g. Dr. Tim's, Fritz Fishless Fuel). A common dose is 4 drops of Dr. Tim's per gallon = about 2 ppm. Never use household ammonia — it has toxic additives.
3. Test every 2–3 days for ammonia, nitrite, and nitrate. Record readings so you can see the pattern.
4. Hold near the target until nitrite appears. As Nitrosomonas grows, ammonia will start dropping and nitrite will rise.
5. Once nitrite is present, redose ammonia to ~2 ppm whenever it falls below ~1 ppm. Don't "chase" an exact number — dose and move on. Don't dose daily; only when it drops.
6. Wait out the nitrite spike. Nitrite can climb to 2–5+ ppm before the second bacteria (Nitrobacter) catches up and drives it back to zero. This is the longest-feeling part.
7. Confirm the cycle with the 24-hour test — see is-my-tank-done-cycling.

The numbers

Common mistakes

• Chasing the exact ppm. Dosing to a perfect 2.0 every day stresses you and the bacteria. Redose only when it drops below ~1 ppm.
• Water changes mid-cycle. They remove the ammonia the bacteria need. Only change water if something spikes past 5 ppm or pH crashes.
• Ammonia-removing products (AmGuard, Ammo-Lock, Prime as a routine) during cycling — they bind the ammonia and starve your bacteria.
• Letting pH crash. Nitrification is acidifying; if pH falls below ~6.5 the cycle stalls. Fix with a 25–50% water change.

When to worry

• Normal: A big nitrite spike (even 5 ppm) that sits high for a week or two. That's the hardest part of the cycle, not a failure.
• Worry: Ammonia or nitrite pinned above 5 ppm for days — that concentration inhibits the very bacteria you're growing. Do a partial water change to bring it under 5.

What's next

Track what each phase should look like in cycling-water-parameters-by-phase, then confirm you're finished with is-my-tank-done-cycling.