How to Cycle a Saltwater Tank: A Beginner's Guide

Why Cycling Your Tank Is the Most Important Step in Reef Keeping

Before you add a single coral or fish, your saltwater aquarium needs to go through a process called the nitrogen cycle. Skipping or rushing this step is the number one reason new reef tanks crash. The good news? Once you understand what's happening and why, it's straightforward to get right.

This guide walks you through everything — from what the nitrogen cycle actually is, to how to test for it, and exactly when your tank is ready for livestock.

What Is the Nitrogen Cycle?

The nitrogen cycle is the biological process by which beneficial bacteria break down toxic waste in your aquarium. Here's the chain:

  1. Ammonia (NH₃) — produced by fish waste, uneaten food, and decaying matter. Highly toxic to fish and coral.
  2. Nitrite (NO₂⁻) — a second type of bacteria converts ammonia into nitrite. Also toxic.
  3. Nitrate (NO₃⁻) — a third bacterial colony converts nitrite into nitrate. Far less toxic, and manageable with water changes and a refugium.

Your goal during cycling is to establish large, stable colonies of these bacteria so they can process waste as fast as it's produced.

What You'll Need Before You Start

  • A filled and running aquarium with a working return pump and heater
  • Pure RODI water — never use tap water, which contains chlorine and chloramines that kill beneficial bacteria
  • A quality dry salt mix
  • An ammonia source
  • Reliable test kits to track ammonia, nitrite, and nitrate levels

Important: Turn your protein skimmer off and remove filter socks for the duration of the cycle. Both will strip beneficial bacteria and organic matter from the water column before your bacterial colonies have a chance to establish. Run your return pump to keep water circulating and oxygenated, but hold off on mechanical and protein filtration until the cycle is complete.

For your water, we strongly recommend making your own RODI water rather than buying pre-made. A home RODI unit pays for itself quickly and ensures you always have pure, 0 TDS water on demand. Our 4 Stage 75GPD RODI System is a great choice for reef keepers — it produces high-purity water suitable for even the most demanding SPS tanks.

For salt, use a quality dry reef salt mix rather than pre-mixed saltwater — it gives you full control over salinity and is more economical for larger tanks. We stock a range of trusted salt mixes including options from Reef Revolution, Tropic Marin, and Aquaforest.

Step-by-Step: How to Cycle Your Tank

Step 1 — Fill with RODI Water and Mix Salt

Fill your tank with freshly made RODI water and add your dry salt mix to reach a salinity of 1.025–1.026 specific gravity (or 35 ppt). Run your return pump and heater and allow the water to reach a stable temperature of 25–26°C before proceeding. Remember — skimmer off, filter socks out.

Step 2 — Add an Ammonia Source

You need ammonia to kick off the cycle. The cleanest and most reliable method is to use Ammonium Chloride by Dr Tim's — it's pure, surfactant-free, and comes with dosing instructions so you can hit exactly 2 ppm without guesswork. Avoid using household ammonia, which often contains additives that can harm your bacteria.

Step 3 — Add Beneficial Bacteria

To significantly speed up your cycle, add a bottle of One & Only Live Nitrifying Bacteria by Dr Tim's on day one. This introduces the exact strains of bacteria needed to process ammonia and nitrite, cutting your cycle time from 4–6 weeks down to as little as 1–2 weeks in many cases. It's one of the best investments you can make when setting up a new reef.

Step 4 — Wait and Test Regularly

Test your water every 2–3 days. For cycling, you'll need three Salifert test kits:

Salifert kits are trusted by reef keepers worldwide for their accuracy and ease of use. You're looking for this progression:

  • Days 1–7: Ammonia rises
  • Days 7–14: Nitrite appears as ammonia-eating bacteria establish
  • Days 14–28: Nitrate appears as nitrite-eating bacteria establish
  • Days 28+: Ammonia and nitrite both drop to 0 ppm — your cycle is complete (sooner if using Dr Tim's One & Only)

Step 5 — Confirm the Cycle Is Complete

Your tank is cycled when:

  • Ammonia reads 0 ppm
  • Nitrite reads 0 ppm
  • Nitrate is present (typically 5–20 ppm)

Dose a fresh shot of Dr Tim's Ammonium Chloride to 2 ppm and retest after 24 hours. If both ammonia and nitrite are back to 0, your bacterial colonies are robust enough to handle a bioload.

Step 6 — Do a Large Water Change Before Adding Livestock

Before adding any fish or coral, do a 30–50% water change to bring nitrate down below 10 ppm. Now is also the time to reinstall your filter socks and turn your protein skimmer back on. This gives your new inhabitants the cleanest possible start.

How Long Does Cycling Take?

A fishless cycle typically takes 4–8 weeks without bacterial supplements. With Dr Tim's One & Only, many hobbyists see a complete cycle in 1–2 weeks. Keeping the temperature at 26°C also helps accelerate bacterial growth.

Common Cycling Mistakes to Avoid

  • Running your skimmer during the cycle — protein skimmers remove the organic compounds that feed your bacterial colonies. Turn it off until the cycle is complete.
  • Leaving filter socks in — filter socks trap and remove bacteria from the water. Take them out for the duration of the cycle.
  • Using tap water — chlorine and chloramines kill the bacteria you're trying to grow. Always use freshly made RODI water.
  • Adding fish too early — even a small ammonia spike can stress or kill fish and coral.
  • Stopping testing — the only way to know your cycle is complete is to test. Don't guess.
  • Using household ammonia — many brands contain surfactants or fragrances that are toxic to bacteria. Use Dr Tim's Ammonium Chloride for a clean, reliable dose.

Ready to Add Coral?

Once your tank is cycled and stable, it's time for the fun part. Start with hardy, beginner-friendly corals like Zoanthids or Soft Corals before moving on to more demanding LPS and SPS species. Browse our full Corals collection to find the perfect first additions for your new reef.

Have questions about cycling or setting up your first reef tank? Get in touch — we're always happy to help.

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