How to Cycle Your Aquarium the Right Way

How to Cycle Your Aquarium the Right Way

The Step Most Beginners Skip (And Shouldn't)

Ask any experienced aquarist what the number one mistake beginners make, and they'll almost certainly say the same thing: not cycling the tank before adding fish. It's the most critical step in setting up a new aquarium, and skipping it is the leading cause of fish loss in new setups.

The good news? Once you understand what cycling actually is and why it matters, it's straightforward to do correctly.

What is the Nitrogen Cycle?

The nitrogen cycle is the biological process by which beneficial bacteria break down toxic waste products in your aquarium water. Here's how it works:

  1. Fish (or other organic matter) produce ammonia (NH3) — a highly toxic compound even at low concentrations.
  2. Nitrosomonas bacteria convert ammonia into nitrite (NO2⁻) — also toxic to fish.
  3. Nitrospira bacteria convert nitrite into nitrate (NO3⁻) — far less toxic and manageable with regular water changes.

The goal of cycling is to establish colonies of these beneficial bacteria in your filter media before you introduce livestock. Without them, ammonia and nitrite will spike to lethal levels.

How Long Does Cycling Take?

A fishless cycle typically takes 4–6 weeks. There's no shortcut — the bacteria need time to colonise your filter media. However, you can speed up the process by:

  • Using a bacterial starter product (e.g., Seachem Stability, Dr Tim's One and Only)
  • Adding filter media or gravel from an established, healthy tank
  • Keeping the temperature at 26–28°C to encourage bacterial growth
  • Maintaining ammonia at 2–4 ppm throughout the cycle

How to Cycle Your Tank: Step by Step

Step 1: Set Up Your Tank Fully

Install your filter, heater, substrate, and any hardscape or plants. Fill with dechlorinated water (use a quality water conditioner like Seachem Prime). Do not add fish yet.

Step 2: Add an Ammonia Source

Beneficial bacteria need ammonia to grow. Options include:

  • Pure ammonia (no surfactants): Dose to 2–4 ppm. This is the most controlled method.
  • Fish food: Add a small pinch daily to decompose and release ammonia.
  • A hardy "cycling fish" (not recommended): Exposes fish to harmful conditions — we advise against this method.

Step 3: Test Your Water Regularly

Use a liquid test kit (not strips — they're inaccurate) to monitor ammonia, nitrite, and nitrate levels every 2–3 days. A quality kit like the API Freshwater Master Test Kit is essential.

Step 4: Watch for the Cycle to Progress

You'll see ammonia rise first, then nitrite will spike as the first bacteria colony establishes. Finally, nitrate will begin to appear as the second colony takes hold. The cycle is complete when:

  • Ammonia reads 0 ppm
  • Nitrite reads 0 ppm
  • Nitrate is detectable (typically 20–40 ppm)

Step 5: Do a Large Water Change and Add Fish

Once ammonia and nitrite both read zero, do a 50% water change to reduce nitrate, then slowly introduce your first fish. Don't add your full stocking list at once — add fish gradually over several weeks to avoid overwhelming your bacterial colony.

Common Cycling Mistakes

  • Using tap water without dechlorinating: Chlorine kills beneficial bacteria. Always use a dechlorinator.
  • Cleaning filter media with tap water: Same issue — rinse filter media in tank water only.
  • Stopping ammonia dosing too early: Keep dosing until the cycle is confirmed complete.
  • Adding too many fish at once: Even a cycled tank can be overwhelmed by a sudden bioload spike.
  • Using test strips: Invest in a liquid test kit for accurate readings.

Speeding Up the Cycle with Plants

Adding fast-growing plants like Hornwort, Water Wisteria, or floating plants during the cycle can help absorb ammonia and nitrite directly, reducing the risk of dangerous spikes. This is one reason planted tanks often cycle faster than bare tanks.

You're Ready — Now What?

A properly cycled tank is the foundation of a healthy, thriving aquarium. Once you're through the cycle, the hard part is done — and the fun begins. Browse our range of live fish, aquatic plants, and aquarium equipment at Chloe's Aquatic Centre to stock your newly cycled tank with confidence.

Back to blog