How to cycle a planted aquarium

What cycling actually is

Fish produce ammonia through their gills and their waste, and leftover food rots into more of it. Ammonia is highly toxic, and in a brand-new tank there is nothing to deal with it. Cycling is the weeks you spend growing two groups of bacteria: the first turns ammonia into nitrite, which is also toxic, and the second turns nitrite into nitrate, which is far less harmful and gets removed by water changes and taken up by plants. Once both colonies are established and living in your filter and substrate, the tank processes waste continuously and quietly. That is a cycled tank, and it is what keeps fish alive.

The three ways to cycle

Fishless cycle, step by step

The safest and most controllable method. No fish suffer while the bacteria build.

1. 1Set the tank up fully and get it warm. Fill it, plant it, run the filter and heater, and hold the temperature around 26 to 28°C. Warmth speeds the bacteria. Dechlorinate the water first, since chlorine kills the very bacteria you are trying to grow.
2. 2Add an ammonia source. Dose bottled ammonia to about 2 ppm, or drop in a small pinch of fish food to rot. This is the food the bacteria colony feeds on. Test and top back up to roughly 2 ppm if it falls.
3. 3Seed it with bacteria. Add a bottled starter culture, or better, a handful of media or a sponge squeezed out from an established, healthy tank. Live seed media is the single biggest shortcut there is.
4. 4Test every few days and wait. Watch ammonia rise then fall as nitrite appears, then watch nitrite fall as nitrate climbs. This nitrite phase is the long one. Keep the ammonia topped up so the colony does not starve.
5. 5Confirm the tank can clear a full dose in 24 hours. Dose ammonia back to 2 ppm and test the next day. When both ammonia and nitrite read zero after 24 hours, the cycle is complete and stable.
6. 6Do a big water change, then stock slowly. Change a large amount of water to bring the built-up nitrate down, then add fish a few at a time so the colony can grow into the new load.

The plant-heavy silent cycle

The planted-tank advantage. Fast-growing plants take up ammonia as food, so a heavily planted tank can skip most of the visible spike.

1. 1Plant heavily from day one. Fill a good share of the tank with fast growers, floating plants, stems and hornwort, which eat ammonia directly. The more plant mass, the less ammonia ever reaches the water.
2. 2Go easy on livestock at first. Add just a few hardy animals, or none, so the waste stays within what the plants can absorb. A shrimp or snail cleanup crew suits this well.
3. 3Test and watch for any spike. Keep testing ammonia and nitrite. In a truly plant-heavy tank they often barely register, which is the whole point, but test so you catch it if the plants can't keep up.
4. 4Grow the stocking in slowly. As the plants establish and the filter bacteria build quietly in the background, add livestock gradually rather than all at once.

Fish-in cycle, safely

If fish are already in the tank, you cannot wait, so the job becomes protecting them while the bacteria catch up.

• Use only a few small, hardy fish, and never a full stocking.
• Test ammonia and nitrite daily. Any reading above zero means a water change now.
• Do partial water changes, often daily, to keep ammonia and nitrite near zero the whole time.
• Feed lightly, since every bit of food becomes ammonia the fish have to survive.
• A dechlorinator that detoxifies ammonia and nitrite buys the fish time between changes.
• Expect it to take longer and demand more of you than a fishless cycle, because you are protecting live fish the entire time.

What the test numbers should do

A liquid test kit for ammonia, nitrite and nitrate is not optional, it is the only way to see the cycle happening. Here is the shape to expect.

1. Week 1Ammonia climbs. Nitrite and nitrate still near zero. The first bacteria are only just waking up.
2. Weeks 2 to 3Ammonia starts dropping as nitrite spikes. This is the halfway mark and the slowest, most patience-testing stage.
3. Weeks 3 to 5Nitrite falls toward zero and nitrate steadily rises. Nitrate going up is the good sign you are waiting for.
4. DoneA fresh ammonia dose reads zero ammonia and zero nitrite 24 hours later. Big water change, then stock slowly.

How to speed it up

• Seed with mature media. A squeezed sponge or a scoop of substrate from a healthy established tank carries the live bacteria and can cut weeks off.
• Use a quality bottled bacteria starter if you have no mature tank to borrow from.
• Keep it warm, around 26 to 28°C, since the bacteria multiply faster in warm water.
• Keep the ammonia fed but not sky-high, since very high ammonia actually stalls the colony.
• Add plenty of surface area, more filter media and porous hardscape give the bacteria more to colonise.
• Don't do water changes during a fishless cycle unless ammonia climbs past about 5 ppm, since changes just slow the feeding.

Common mistakes

• Chasing a fast cycle and adding a full stocking at once, then crashing it.
• Rinsing filter media in tap water, which chlorine sterilises, wiping out the colony.
• Doing constant water changes during a fishless cycle, which starves the bacteria.
• Trusting a bottle of bacteria to make the tank instantly safe without testing.
• Reading a nitrite reading as failure, when it is actually the normal middle stage.
• Skipping the test kit entirely, which is the only way to actually know where you are.

Once you're cycled

• Understand the numbers: pH, KH, GH and more
• Keep it healthy: the maintenance routine
• Pick fast plants that speed a silent cycle
• Choose hardy first fish
• Something already going wrong? Fish & shrimp health
• Plants looking off? Deficiency ID
• Diagnose new-tank algae