What's the best way to cycle a saltwater tank?
When: At the start of cycling, before you choose your approach
Short answer: The two methods worth your time are fishless cycling with dosed ammonia (precise, humane, reliable) and the same thing sped up with bottled bacteria (as fast as 5–14 days). Live rock also works and adds biodiversity but risks pests. Never cycle with a fish, and never use household ammonia.
The details — your realistic choices
1. Fishless cycling with dosed ammonia — the recommended default. You add aquarium-grade ammonium chloride to feed the bacteria directly. Total control, no animal harmed, and you can build a large colony by dosing to a target. Takes 3–6 weeks on its own. Full steps: fishless-cycle-with-ammonia.
2. Fishless + bottled bacteria — the fast path. Add a bottle of live nitrifying bacteria (Dr. Tim's One & Only, Fritz TurboStart 900, Brightwell MicroBacter) plus an ammonia source. This seeds the colony instead of waiting for it to appear, cutting cycling to roughly 5–14 days. Full steps: fishless-cycle-with-bottled-bacteria.
3. Live rock cycling — biodiversity, with a pest tax. Cured or uncured live rock brings existing bacteria and life (coralline algae, copepods). Uncured rock's die-off supplies its own ammonia. Downsides: it can introduce hitchhiker pests (aiptasia, mantis shrimp), uncured rock can spike ammonia hard, and good live rock is increasingly hard to source. 2–8 weeks depending on cure state.
4. Seeded media from an established tank — near-instant, if you can get it. Transferring mature rock, sand, or filter media can cycle a tank in 0–7 days. You need access to a healthy established system, and you inherit whatever pests/algae/disease it carries.
Methods to avoid:
- Fish-in cycling — cruel and risky; the fish suffers weeks of ammonia/nitrite burns.
- Dead shrimp / fish food ("ghost feeding") — works but slow (6–8+ weeks), messy, and hard to control.
The numbers
| Method | Typical timeline |
|---|---|
| Seeded mature filter media | 0–7 days |
| Fishless + Fritz TurboStart 900 | 5–7 days |
| Fishless + Dr. Tim's + ammonia | 7–14 days |
| Live rock (cured) + bottled bacteria | 7–14 days |
| Fishless, dosed ammonia only (no bottled bacteria) | 3–6 weeks |
| Live rock (uncured) | 4–8 weeks |
| Dead shrimp / fish food | 6–8+ weeks |
Common mistakes
- Household ammonia. It contains surfactants and additives that are toxic to the tank. Use only reagent-grade or aquarium-specific ammonium chloride.
- Expecting bottled bacteria to work without an ammonia source. The bacteria starve and die if there's nothing for them to eat.
- Assuming pricier = better. A patient dosed-ammonia cycle with a cheap API test kit works perfectly; you're mostly paying for speed.
When to worry
- Normal: Choosing dosed-ammonia and watching nothing happen for a week. Bacteria are multiplying invisibly.
- Worry: Uncured live rock driving ammonia past 5 ppm — that level actually inhibits the bacteria. Do a partial water change to bring it down.
What's next
Ready to execute: fishless-cycle-with-ammonia (patient) or fishless-cycle-with-bottled-bacteria (fast).
- Any plan that puts a fish in an uncycled tank
- Household/cleaning ammonia (contains toxic surfactants) instead of aquarium-grade
- Reef Tank Cycling Research (internal)
- Bulk Reef Supply — How to Cycle a Saltwater Tank