Reef Tank Nitrogen Cycle: Getting It Right First Time

The Boring Bit That Saves Everything

Cycling is the most tedious part of setting up a reef tank. But it's the make-or-break step that determines whether the first fish live or die.

What's Actually Happening

When organic matter breaks down — fish waste, uneaten food, die-off from live rock — it produces ammonia, which is toxic even in small amounts.

Beneficial bacteria colonise the tank and process ammonia in stages:

1. Ammonia (deadly) → converted by nitrifying bacteria to...
2. Nitrite (still toxic) → converted by a second group to...
3. Nitrate (relatively harmless at low levels)

This bacterial population takes time to build up. That's the cycle.

How to Cycle a Tank

The Ammonia Source Method

1. Set up the tank fully — rock, sand, salt water, equipment running
2. Add an ammonia source: Dr Tim's ammonia, a raw prawn left to decompose, or fish food sprinkled in daily
3. Test ammonia, nitrite, and nitrate every 2-3 days
4. Wait

What to Expect

Week 1-2: Ammonia rises. Normal — the process is starting.

Week 2-4: Ammonia drops, nitrite spikes. First bacteria colony is working.

Week 3-6: Nitrite drops, nitrate rises. Nearly there.

Cycle complete: Ammonia at 0, nitrite at 0, nitrate present. Dosing ammonia to 2ppm drops to zero within 24 hours.

Can It Be Sped Up?

• Bottled bacteria (Dr Tim's One and Only, Fritz TurboStart) — can cut cycling to 2-3 weeks
• Mature filter media from an established tank
• Live rock from an established system

There's no magic chemical that skips the cycle entirely, despite what some products imply.

Common Mistakes

• Adding fish too early — wait until both ammonia AND nitrite are consistently zero
• Cleaning during cycling — leave the tank alone; the goal is growing bacteria, not removing them
• Using tap water — chloramine kills the bacteria being cultivated. Use RODI from the start
• Testing too infrequently — every 2-3 days catches the cycle completion at the right time

After the Cycle

Start slowly:

1. Clean-up crew first — hermit crabs, snails
2. First fish 1-2 weeks later — something hardy like a clownfish pair
3. First corals after 2-4 more weeks — soft corals first (mushrooms, zoanthids)
4. Add livestock gradually — one or two additions per month maximum

Each addition increases bioload, and the bacteria need time to scale up. Rushing causes mini-cycles that stress everything.

What methods have worked well for cycling? Any tips for making the wait more bearable?