Cycling a new tank.

What cycling means

Fish produce ammonia, and ammonia is poisonous to them even at low levels. In an established tank, two families of bacteria living in the filter deal with it: the first turns ammonia into nitrite, which is also harmful, and the second turns nitrite into nitrate, which is mild and which you remove with water changes. That chain is the nitrogen cycle.

A brand-new tank has none of those bacteria yet. Cycling means growing them before the fish arrive, so that the tank can handle waste from the first day fish are in it.

Cycling without fish, step by step

1. Set the tank up completely. Substrate, decorations, plants if you have them, filter running, heater set to about 26 to 28 degrees. Bacteria grow faster in warm water. Add dechlorinator to the water first, since chlorine clears out the bacteria you're growing.
2. Add a source of ammonia. The tidiest way is a bottled ammonia sold for the purpose, dosed to about 2 to 4 parts per million on your test kit. A pinch of fish food every day does the same job more slowly as it breaks down.
3. Test every two or three days. You need a kit that measures ammonia, nitrite and nitrate. For the first week or two you'll see ammonia only. Then nitrite appears and ammonia starts to fall, which means the first bacteria have arrived.
4. Keep feeding the cycle. When ammonia drops to zero, dose it back up. The bacteria need a steady supply to keep multiplying.
5. Wait for nitrite to fall. Nitrite often climbs high and sits there for a while. This is the slow part. When it finally drops and nitrate starts climbing, the second bacteria have arrived.
6. The finish line. The tank is cycled when a full dose of ammonia is turned into zero ammonia and zero nitrite within 24 hours. Do a large water change to bring nitrate down, match the temperature, and add your first fish.

Making it faster

A bottled bacteria starter from the shop adds the bacteria directly and usually brings the whole process down to a week or two. Follow the bottle, keep dosing ammonia, and keep testing. It still needs confirming with the test kit before fish go in.

Faster still is seeding from an established tank. A squeeze of a mature filter sponge into your new filter, or a handful of gravel from a running tank in a mesh bag, carries a working colony across. If you buy your fish from us, ask. We can often help with a piece of mature media, and it can take a cycle from weeks to days.

If the fish are already in

It happens, and the tank can still be cycled with them in it. It needs a little more care from you for a few weeks.

• Keep the fish few and feed lightly, once a day, only what disappears in a minute.
• Test every day. Any reading of ammonia or nitrite above a trace means a 25 to 30 percent water change that day, with dechlorinator.
• Use a dechlorinator that also detoxifies ammonia for a day or two. It buys the bacteria time.
• Add a bacteria starter. This is the situation it was made for.
• Hold off on adding more fish until ammonia and nitrite have read zero for a week.

Looking after the bacteria once you have them

Almost all of the colony lives in the filter media, with some on the gravel and surfaces. Rinse filter sponges in a bucket of old tank water on water-change day, and only when they're clogged. Tank water keeps the colony alive, where tap water would wash it out. Replace media one piece at a time so the rest of the colony carries on, and keep the filter running around the clock. The bacteria will get through a few hours of load shedding as long as the media stays wet.

How you know it's ready

Ammonia zero. Nitrite zero. Nitrate present and rising. Those three together, held for a couple of days, are the whole test. If you want a second opinion, message us a photo of your test results and we'll tell you what we see.