How to Cycle a New Aquarium (Fishless, Step by Step)

The most common way a first aquarium goes wrong is this: tank set up on Saturday, fish added on Sunday, and over the following three weeks the fish die one at a time while the water looks completely clear. This is New Tank Syndrome, and it is entirely preventable.

What cycling actually is

Fish excrete ammonia, continuously, through their gills as well as their waste. Ammonia is acutely toxic. It burns gill tissue at concentrations well under 1 ppm. In an established tank you never see it, because two groups of bacteria living in the filter media and on every surface deal with it:

1. 1Ammonia-oxidising bacteria convert ammonia (NH₃) into nitrite (NO₂⁻). Nitrite is still toxic: it binds to haemoglobin and stops blood carrying oxygen.
2. 2Nitrite-oxidising bacteria convert nitrite into nitrate (NO₃⁻), which is comparatively harmless and gets removed by your weekly water change.

Those colonies take weeks to establish, and they grow in proportion to the food available. A brand new tank has essentially none. Adding fish first means the fish are the ammonia source while the colony builds, which is why they die, and why the ones that survive often develop gill damage that shortens their lives.

What you need before you start

• A liquid test kit reading ammonia, nitrite, nitrate and pH. This is non-negotiable, because strips are too imprecise below 0.5 ppm, which is exactly the range you're making decisions in.
• A pure ammonia source: household ammonia with no surfactants, perfumes or colourants. Shake it. If it foams, don't use it.
• Dechlorinator, for the initial fill and every water change after.
• A filter, running continuously, with biological media in it.
• A heater. Bacteria establish considerably faster at 26–28°C than at room temperature.

The process

Days 1–3: set up and dose

Fill the tank, dechlorinate, and get the filter and heater running. Set the heater to 27°C. Add hardscape, substrate and any plants now. Plants help, and you want the surfaces in place for bacteria to colonise.

Then dose ammonia to 2 ppm and test to confirm you're actually at 2. Dosing is not intuitive, because the strength of household ammonia varies, so add a small amount, test, and work up. Write down the volume that got you to 2 ppm, because you'll reuse that figure repeatedly.

Weeks 1–2: waiting for nitrite

Test ammonia and nitrite daily and write the numbers down. For the first week or so, nothing appears to happen. Then ammonia starts falling and nitrite appears, usually somewhere between day 8 and day 14. That first nitrite reading is the milestone: it means colony one is established.

Re-dose ammonia back to 2 ppm each time it drops below about 0.5 ppm. You are feeding the colony; if you let the food run out, it stops growing.

Weeks 2–5: the nitrite spike

This is the long part, and where most people give up. Nitrite climbs, often off the top of the test scale, and sits there. It can look completely static for ten days. It is not static. Colony two is building, it's just slower-growing than colony one.

If nitrite is off the chart and hasn't moved in a fortnight, a 50% water change with dechlorinated water dilutes it back into a readable range and frequently restarts visible progress. You will not lose the colony, which lives on surfaces, not in the water.

The finish line

Dose ammonia to 2 ppm in the evening. Test 24 hours later. You are cycled when you read:

Both zeros, in 24 hours, repeated twice on consecutive days. Ammonia at zero with nitrite still reading anything at all means you are not finished. That's the single most common false finish.

Then do a large water change, 50–80%, to bring nitrate down before adding fish. Add fish within a day or two, and add them in small groups: the colony is sized to 2 ppm of ammonia a day, and it needs a few days to scale up to a full stocking.

How to make it faster

• Seed the filter. A handful of media from an established, healthy tank can cut the cycle to under a fortnight. This is by far the biggest lever, and a local fishkeeper or a decent aquatics shop will often hand some over.
• Keep it at 27°C. Colder tanks cycle appreciably slower.
• Keep pH above 7. Below about 6.5 the bacteria slow markedly, and below 6 the cycle can stall entirely.
• Aerate well. Both bacterial groups are aerobic; an air stone or a decent surface ripple genuinely speeds things up.

Bottled bacteria products are a mixed bag. Some genuinely contain the right species and help; many contain species that don't survive shelf storage. If you use one, treat it as a possible head start rather than a substitute for testing. The test kit is what tells you the truth.

If you already have fish in an uncycled tank

Don't panic, and don't take them back out into another uncycled container. Test ammonia and nitrite daily, and do a water change of whatever size brings both under 0.25 ppm, often 50% daily at the peak. Feed lightly, every other day; uneaten food is a direct ammonia source. Dechlorinate everything. It's more work and harder on the fish, but it's survivable if you're consistent for the three or four weeks it takes.

The unglamorous truth is that cycling is mostly waiting and writing numbers down. But the six weeks you spend on it are the difference between a tank that runs itself for years and one that stays a problem.