How to Cycle a Fish Tank
(Complete Beginner's Guide)

What Is "Cycling" an Aquarium?

Cycling an aquarium means establishing a colony of beneficial bacteria inside your filter that will continuously break down fish waste. Without this bacterial colony, toxic compounds build up rapidly and kill your fish — a problem so common it has its own name: New Tank Syndrome.

The process follows what's called the nitrogen cycle. Fish produce ammonia levels through their waste and breathing. Left unchecked, ammonia is lethal. Beneficial bacteria (specifically Nitrosomonas) convert ammonia into nitrite — also toxic. A second group of bacteria (Nitrospira) then convert nitrite into nitrate, which is relatively harmless at low levels and removed through regular water changes.

Why Cycling Matters

A brand new aquarium has no beneficial bacteria. When you add fish immediately, ammonia spikes within 24–48 hours. Even at 0.25 ppm (parts per million), ammonia begins to stress fish. At 2–4 ppm it causes chemical burns to gills and internal organs. Most fish die within a week in an uncycled tank, and beginners often blame the fish or the equipment — not realising the tank simply wasn't ready.

What You Need Before You Start

• A water test kit — liquid test kits (like the API Master Test Kit) are far more accurate than strips. You need to test for ammonia, nitrite, nitrate, and pH.
• An ammonia source — pure ammonia (no surfactants/fragrances), fish food, or a raw prawn/shrimp placed in the tank.
• A running filter — the bacteria live in your filter media, not the water.
• A running heater — bacteria grow faster at 25–28°C (77–82°F).
• Dechlorinator (water conditioner) — chlorine kills bacteria. Treat all tap water before adding it to your tank.

How to Cycle Your Tank: Step by Step

1. 1
   Set up your tank fully

   Fill the tank with dechlorinated tap water. Install your filter, heater, and any decorations. Turn everything on. Do not add fish yet.

2. 2
   Add your ammonia source

   Dose pure ammonia to reach 2–4 ppm. If using fish food or a raw prawn, add a small pinch or one small prawn and leave it to decompose. Test the water after 24 hours.

3. 3
   Wait and test every 2–3 days

   Check ammonia, nitrite, and nitrate levels regularly. In the first 1–2 weeks, you should see ammonia starting to drop as nitrite begins to rise. This means your first bacteria colony is establishing.

4. 4
   Top up ammonia as needed

   Keep ammonia between 1–2 ppm throughout the cycle. If it drops to 0 before nitrite has risen, add more. The bacteria need a constant food source to multiply.

5. 5
   Watch for nitrate to appear

   Around week 3–5, you'll notice nitrate starting to climb while nitrite begins to fall. This means the second bacteria colony is growing. You're in the home stretch.

6. 6
   Confirm the cycle is complete

   The cycle is done when: you dose ammonia to 2 ppm, check 24 hours later, and both ammonia AND nitrite read 0 ppm while nitrate has risen. This means both bacteria colonies are fully established.

7. 7
   Do a large water change, then add fish

   Before adding fish, do a 50% water change to lower nitrate below 20 ppm. Now you can safely add your fish — slowly and in small numbers over several weeks.

What Parameters to Look For

Can You Speed Up the Cycle?

Yes — within reason. Here are legitimate methods that actually work:

• Use established filter media. Ask a friend or local fish store for a handful of used filter media. Seeding your new filter with existing bacteria can cut cycling time to 1–2 weeks.
• Raise the temperature. Bacteria reproduce faster in warmer water. Keeping the tank at 28–30°C (82–86°F) during the cycle (without fish) can accelerate growth.
• Use a bacterial supplement. Products like Seachem Stability or API Quick Start contain live bacteria cultures. They're not magic, but they can shave 1–2 weeks off the process when used alongside proper ammonia dosing.
• Add some substrate or decor from an established tank. Gravel and decorations harbour beneficial bacteria too.

What About Cycling With Fish?

Historically, people cycled tanks with fish already in them because there was no alternative. This is now considered inhumane by most aquarists, as it subjects fish to toxic water for weeks. However, if you've already bought fish before reading this (it happens), you can manage the cycle with fish in the tank by:

• Doing daily 25–30% water changes to keep ammonia and nitrite below 0.25 ppm
• Adding a bacterial supplement like Seachem Stability daily for the first two weeks
• Not overfeeding — uneaten food adds to ammonia load
• Keeping the temperature stable and the filter running constantly
• Testing every single day until the cycle is complete

How to Know Your Cycle Is Done

The definitive test: dose your tank to 2 ppm ammonia. Check again exactly 24 hours later. If both ammonia and nitrite read 0 ppm and nitrate has risen, your cycle is complete. The bacteria colonies are large enough to process a full dose of ammonia in a single day — exactly what they'll need to do once your fish are producing waste continuously.

Quick Summary

Cycling an aquarium takes patience but it's not complicated. Set up your tank, add an ammonia source, test regularly, and wait for ammonia and nitrite to both hit 0 ppm with rising nitrate. That's it. Do this once and you'll never have to deal with New Tank Syndrome again. Your fish will thank you for it.