How long does it take to cycle a tank?

A fishless cycle usually takes about two to six weeks. Seeding the filter with media from an established tank, or using a bottled bacteria starter, can shorten it considerably.

What is the nitrogen cycle?

Fish waste and leftover food break down into ammonia, which is toxic. One group of bacteria converts ammonia to nitrite (also toxic), and a second converts nitrite to nitrate (much less toxic), which you remove with water changes.

Do I really need a liquid test kit?

Yes. You cannot see ammonia, nitrite or nitrate. A liquid test kit is the only way to know where the cycle stands and when it is safe to add fish. Paper strips are less reliable for cycling.

How to Cycle a New Fish Tank (Fishless Cycling, Step by Step)

Most new-tank fish deaths are not bad luck. They are "new tank syndrome": adding fish before the tank can process their waste. Cycling is how you build the invisible cleanup crew of bacteria first, so the water stays safe. Doing it without fish, a fishless cycle, is the humane and reliable way.

The nitrogen cycle in plain English

Fish produce waste, and leftover food decays. Both release ammonia, which is toxic even in small amounts. In a healthy tank, one colony of bacteria eats ammonia and turns it into nitrite, which is also toxic. A second colony turns nitrite into nitrate, which is far less harmful and which you remove with regular water changes. Cycling simply means growing those two colonies before any fish move in.

Fishless cycling, step by step

Set up and run the tank with dechlorinated water, filter and heater going.
Add ammonia to about 2 to 4 ppm, using pure ammonia or a little decaying fish food.
Test regularly. Ammonia falls and nitrite rises as the first colony grows.
Keep waiting. Nitrite falls as the second colony converts it to nitrate.
Confirm: a fresh ammonia dose reaching 0 ammonia and 0 nitrite in 24 hours means you are cycled.
Big water change, then add fish gradually.

Speeding it up (safely)

You do not have to start from zero. Squeezing filter media from an established, healthy tank, or adding a bottled nitrifying-bacteria starter, seeds the colonies and can cut a multi-week cycle down to days. Warmth (around 78 to 82 F) and good oxygenation also help the bacteria multiply.

Once you are cycled

A cycled tank is ready for a careful stocking plan, not a full house of fish at once. Size the tank first with the aquarium volume calculator, then build a compatible list with the fish stocking calculator. And skip the old shortcut while you are at it: see why the 1 inch per gallon rule is a myth.

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