How to Cycle a New Aquarium (Freshwater & Saltwater)

Cycling your aquarium is the single most important step before adding any livestock. Skip it and you risk ammonia poisoning, fish loss, and endless frustration. Here’s exactly how to do it.

What Is the Nitrogen Cycle?

The nitrogen cycle is the biological process that converts toxic ammonia (from fish waste and uneaten food) into less harmful compounds:

Ammonia → Nitrite → Nitrate

This is driven by beneficial bacteria (Nitrosomonas and Nitrospira) that colonize your filter media, substrate, and live rock. A fully cycled tank converts ammonia to nitrite and nitrite to nitrate rapidly.

How Long Does Cycling Take?

• Standard fishless cycle: 4–8 weeks
• With bottled bacteria (e.g., Seachem Stability, Fritz TurboStart): 1–2 weeks
• With established filter media from a cycled tank: days to 1 week

Fishless Cycling (Recommended)

1. Set up the tank with all equipment running
2. Add an ammonia source: pure ammonia (unscented), fish food, or a raw prawn
3. Dose ammonia to 2–4 ppm
4. Test every 2–3 days with an ammonia, nitrite, and nitrate test kit
5. When you see nitrite rise then fall, and nitrate appears, you’re near the end
6. When you can dose 2 ppm ammonia and it converts to nitrate within 24 hours — you’re cycled

Testing Throughout

A quality test kit is non-negotiable. We carry the API Master Test Kit in our Aquarium Supplies — it covers ammonia, nitrite, nitrate, and pH in one kit.

Saltwater & Reef Cycling

For reef tanks, use live rock (which carries beneficial bacteria) and quality salt mix. Cycle with ammonia or a small piece of raw shrimp. Do not add corals until the cycle is fully complete.

After the Cycle

• Do a large water change (50%+) to lower nitrates before adding livestock
• Add fish slowly — don’t fully stock all at once
• Continue testing regularly for the first few months

Need supplies? Browse our full range of test kits, water conditioners, and cycling products in our Aquarium Supplies store.