How to Cycle a New Fish Tank: NZ Beginner's Guide

The quick answer: "cycling" means growing the beneficial bacteria in your filter that turn toxic fish waste (ammonia) into less harmful nitrate. A brand-new tank has none of this bacteria, so adding fish straight away poisons them. The safest method is a fishless cycle: add an ammonia source, test the water, and wait (usually 4–6 weeks) until the tank can process ammonia and nitrite to zero. Then it's safe for fish.

The nitrogen cycle in plain English

Fish produce ammonia (toxic). One group of bacteria converts ammonia to nitrite (also toxic). A second group converts nitrite to nitrate (much less harmful, removed by water changes). Growing both bacteria colonies is "the cycle". Until it's established, ammonia and nitrite spike and can kill fish.

You'll need a liquid test kit

A liquid test kit for ammonia, nitrite and nitrate is essential — you literally cannot see the cycle without it. Test strips are less accurate; a liquid kit is worth it.

Fishless cycling (recommended)

1. Set up the tank fully (filter running, dechlorinated water). See how to set up a first aquarium in NZ. 2. Add an ammonia source — a few flakes of food or bottled pure ammonia — to feed the growing bacteria. 3. Test regularly. Ammonia rises, then falls as bacteria grow; nitrite rises, then falls; nitrate appears. 4. Cycle is done when you can add ammonia and, within 24 hours, both ammonia and nitrite read zero and you see nitrate. This usually takes 4–6 weeks. 5. Do a water change to lower nitrate, then add fish slowly.

Speeding it up

• Seed the filter with media or gravel from an established, healthy tank (a great Kiwi fish-club or friend's tank) — this adds bacteria instantly and can halve the time.
• Bottled bacteria products can help but vary in quality.
• Keep the heater on (warmth speeds bacteria) and don't clean the filter media in tap water (chlorine kills the bacteria) — rinse it in old tank water only.

Fish-in cycling (only if unavoidable)

If you already have fish in an uncycled tank, you must do frequent partial water changes to keep ammonia and nitrite low while the cycle catches up, and add a dechlorinator that detoxifies ammonia. It's stressful for the fish, so fishless cycling is far kinder.

Shop related categories at PetMall

Looking for test kits, filters and water conditioner in New Zealand? Browse the PetMall fish range for current options and nationwide delivery.

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Related reading

• How to Set Up a First Aquarium NZ
• Aquarium Water Changes NZ
• Aquarium Filter Types NZ

References

• PetMD (veterinary-reviewed — Jessie Sanders DVM, DABVP Fish Practice) — New Tank Syndrome in Aquarium Fish: ammonia and nitrite are highly toxic during the 4–6 week nitrogen cycle, checked 2026-07-11: https://www.petmd.com/fish/conditions/systemic/new-tank-syndrome-fish
• Ornamental Aquatic Trade Association (OATA) — How to test water quality in your freshwater aquarium, checked 2026-07-11: https://ornamentalfish.org/what-we-do/advice-information/care-sheets/caresheets-tropical-freshwater-fish/how-to-test-water-quality-in-your-freshwater-tank-aquarium/

Important notice

*General fishkeeping information for NZ owners. If fish show distress during cycling, do an immediate partial water change and seek advice from a NZ aquatics specialist.*