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.

The bacteria live on surfaces, mostly in the filter media, not in the water. That is why a new filter needs weeks, why you never rinse filter media under chlorinated tap water, and why a piece of media from an established tank is the best shortcut there is. For the arithmetic of how much ammonia a stocked tank actually produces, see what bioload means in an aquarium.

Fishless cycling, step by step

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

How much ammonia to add

Use ammonia with nothing else in it: no scent, no surfactant. Shake the bottle; if it foams, it has soap in it and is not safe. Household ammonia is usually sold at 5 to 10 percent. A 10 percent solution carries about 100 mg of ammonia per mL, and 1 ppm is 1 mg per liter, so the dose is simple arithmetic. Here is the starting amount to reach 2 ppm:

Those are for the nominal volume. Real water volume is lower once substrate and decor are in, often by 10 to 15 percent, so start with a little less, test after an hour of circulation, and top up. Overshooting is the more common mistake: above about 5 ppm the bacteria you are trying to grow slow down. If you would rather not handle ammonia at all, bottled ammonium chloride sold for fishless cycling is dosed by the label.

What the readings look like, week by week

The nitrite phase is the long one and the one that tests patience. It is normal for nitrite to read at the top of the color chart for a week or more. It will fall. A cycle is done when both ammonia and nitrite clear within a day, not when the tank has run for a certain number of weeks.

When the cycle stalls

• Ammonia too high. Readings stuck above about 5 ppm slow the bacteria. Do a partial water change to bring it back to 2 to 3 ppm.
• pH crash. Nitrifying bacteria use up carbonate hardness, and as it runs out pH falls. Below about 6.5 the process slows sharply, and in very soft water it can stop. Test pH; a water change usually restores it.
• Nitrite stuck high for weeks. Very high nitrite can itself slow the second colony. A 50 percent water change often gets things moving again.
• Chlorine or chloramine. Every top-up and water change needs dechlorinator. Untreated tap water can kill a colony that took weeks to grow.
• Too cold. The bacteria grow faster in warm water. Running the tank at 78 to 82 F while cycling helps, then set it to your fish's range before stocking.

Speeding it up (safely)

You do not have to start from zero. Squeezing filter media from an established, healthy tank into the new one, or better, putting a piece of that media into the new filter, seeds the colonies and can cut a multi-week cycle down to days. Bottled nitrifying-bacteria starters vary in quality but some genuinely help. Warmth and good oxygenation help the bacteria multiply. Live plants also absorb some ammonia, which smooths the early weeks.

If you already have fish in the tank

This is a fish-in cycle, and it is common when someone brings fish home from the store the same day as the tank. It works, but you are managing toxins daily until the bacteria catch up. Feed very lightly, once a day or less. Test ammonia and nitrite every day. Whenever either reads above about 0.25 to 0.5 ppm, change 25 to 50 percent of the water with dechlorinated water at the same temperature. A water conditioner that binds ammonia buys some margin between changes. Do not add more fish until the tests have read 0 ammonia and 0 nitrite for a week.

Once you are cycled

A cycled tank is ready for a careful stocking plan, not a full house of fish at once. The colony is sized for the ammonia you were dosing, so add fish in small groups, a week or two apart, and test after each addition. Size the tank first with the aquarium volume calculator, then build a compatible list with the fish stocking calculator. Hardy schooling fish such as zebra danios or harlequin rasboras are good first additions; sensitive bottom dwellers such as corydoras go in once the tank has been stable for a few weeks. And skip the old shortcut while you are at it: see why the 1 inch per gallon rule is a myth.