How to Cycle a New Aquarium (Fishless Cycle, Step by Step)

The most common way new fish die is "new tank syndrome": fish go into a brand-new tank before the good bacteria are established, and their own waste poisons them. The fix is cycling the tank first. It takes patience, usually a few weeks, but it's the foundation of every healthy aquarium.

The nitrogen cycle in plain words

1. Fish waste and leftover food make ammonia, which is toxic.
2. One group of beneficial bacteria turns ammonia into nitrite, which is also toxic.
3. Another group turns nitrite into nitrate, which is far less harmful and is removed with water changes and taken up by plants.

These bacteria live mostly in your filter media and on surfaces. A new tank has almost none. Cycling grows them.

Fishless cycling, step by step

1. Set up the tank completely: substrate, decor, filter, heater if used. Treat tap water with a dechlorinator.
2. Add an ammonia source: pure bottled ammonia (no soaps or perfumes) dosed to a few parts per million, or fish food left to rot. Follow a reliable guide or the product's instructions for the dose.
3. Add bacteria if you can: a bottled bacteria product, or filter media or gravel from an established, healthy tank, can speed things up a lot.
4. Test every few days with a liquid test kit for ammonia, nitrite and nitrate.
5. Watch the stages: ammonia rises, then falls as nitrite rises; then nitrite falls as nitrate rises.
6. Keep feeding ammonia as it's used up, so the bacteria keep growing.
7. It's cycled when the tank can turn your ammonia dose into zero ammonia and zero nitrite within about 24 hours, with nitrate showing up.
8. Do a big water change to bring nitrate down, then add fish, a few at a time.

How long?

Often four to six weeks without seeded media; faster with it. Warmer water speeds it up. Don't rush it.

Mistakes to avoid

• Cleaning the filter media under tap water. Chlorine kills the bacteria. Rinse in old tank water.
• Adding lots of fish at once after cycling. Add a few, then wait.
• Overfeeding new fish.

After it's cycled

Keep up regular water changes, test now and then, and add new fish slowly, after quarantine.

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