The complete beginner's guide to cycling a new aquarium (fishless, step-by-step)

If you've just set up your first tank, do not add fish yet. The most common reason new fish die in the first month is going into water that hasn't cycled. Here's what that means and exactly how to do it — no fish harmed, about 3–6 weeks of patience.

What cycling is. Fish constantly produce ammonia, which is toxic. In an established tank two groups of bacteria handle it: one turns ammonia into nitrite (also toxic), the next turns nitrite into nitrate (mostly harmless at low levels, removed by water changes). Cycling means growing those colonies before fish arrive. (New to the terms? See our glossary entry on the nitrogen cycle.)

What you need. Pure ammonia (ingredient list reads only ammonia and water — no surfactants or perfume); a liquid test kit for ammonia, nitrite, and nitrate (strips are wildly inaccurate); your filter running 24/7; and a heater around 78–80°F so bacteria grow faster.

Step by step.

1. Dose ammonia to about 2 ppm. Note how much it took so you can repeat it — that's your bacteria's food.
2. Test daily. The first week or two, nothing changes. Normal.
3. Ammonia starts falling and nitrite appears — first colony is working. Re-dose ammonia to ~2 ppm whenever it hits 0.
4. Nitrite falls and nitrate appears — second colony online. Nitrite is the slow part, often a couple of weeks.
5. You're cycled when a fresh ~2 ppm dose reads 0 ammonia and 0 nitrite within 24 hours, with nitrate present. Do a 50%+ water change to drop nitrate, then add fish.

Speed it up with a handful of established media, gravel, or a sponge from a healthy, disease-free tank — that's a live transplant of the colonies. Bottled bacteria can help but treat it as a head start, not magic.

Mistakes that reset you: rinsing filter media in chlorinated tap water (kills the colonies — rinse in old tank water), chasing the nitrite spike with water changes (let it ride during a fishless cycle), and adding fish "to test."

Once cycled, stock slowly and check compatibility in the Tank Builder and species database before you buy.

Your turn: post your day-by-day numbers if you're mid-cycle and want a second opinion, or tell us what stalled your cycle so the next beginner can find the fix.