Cycling a marine tank is the least glamorous part of the hobby, and the part that decides how the next few years go. Do it properly and your first fish settle into a stable, forgiving system. Rush it and you spend months chasing ammonia spikes, nuisance algae and losses you cannot explain. Here is the complete checklist, from the first bucket of salt water to the day livestock finally goes in.
Set the tank up properly before anything else
Mix RO water with a quality marine salt to a specific gravity of around 1.024–1.026, check it with a refractometer rather than a swing-arm hydrometer, and bring the tank to 24–26°C. Get the heater, powerheads and return pump running so the water is moving properly around the rock. If you plan to run a skimmer, you can leave it on during the cycle; it will pull out some organics but will not stop nitrifying bacteria from establishing.
Choose your rock and sand now, because you cannot easily change them later. Live rock or a mix of live and dry rock gives you the widest range of surfaces for bacteria to colonise. Keep the lights off, or on for only a few hours a day, to limit the inevitable diatom bloom that arrives in the first fortnight. There should be no fish, no snails and no crabs in the tank at this stage.
What to test, and how often
Nitrification is a chemical process, so you need numbers, not opinions. Buy liquid test kits rather than strips — strips are too coarse to tell the difference between 0.25 and 2 ppm of ammonia, and that difference matters.
- Ammonia — the fuel for the first stage of the cycle
- Nitrite — the intermediate product, and the one that keeps people waiting longest
- Nitrate — the end product, and proof the cycle has actually finished
- pH and carbonate hardness (KH) — nitrification consumes alkalinity, so pH can sag mid-cycle
- Salinity and temperature — keep both steady; swings slow bacteria down
Test at roughly the same time each day and write the results down in a notebook or spreadsheet. A short log turns a confusing month into a readable curve, and it is far easier to see that things are moving than to rely on memory.
Choosing an ammonia source
Liquid ammonium chloride
The cleanest option. Dose to bring ammonia to around 1–2 ppm and let the bacteria do the rest. If you use a household ammonia instead, make sure it contains no surfactants, perfumes or dyes — shake the bottle and if it foams, do not put it near your tank.
Fish food or a prawn
Both work, and both are messier. You cannot control the dose accurately, and rotting food adds phosphate and organics that feed algae. If you go this route, use less than you think you need and accept that you may never see a neat, readable ammonia peak.
What you must not do is use a live fish as the ammonia source. It is stressful for the animal, it is hard to control and, if the cycle stalls, the fish suffers first.
Bacteria products that actually help
Bottled nitrifying bacteria can genuinely shorten a cycle, particularly in a tank filled with dry rock and dry sand. They do not replace the process; they give it a head start. Treat the label claims with a healthy scepticism and expect weeks, not days.
The most reliable inoculation is still something alive. A few kilos of well-cured live rock, a bag of filter media from an established tank, or a cup of sand from a healthy system will seed bacteria far faster than anything from a bottle. Keep that material warm, wet and out of the air — drying out kills the bacteria you have just paid for or borrowed.
The timeline, week by week
Every tank is different, so treat this as a shape rather than a schedule.
- Days 1–3: dose ammonia to 1–2 ppm. It will sit there, apparently doing nothing.
- Weeks 1–2: ammonia starts to fall. Nitrite appears and climbs.
- Weeks 2–4: nitrite peaks, sometimes off the chart. Top ammonia back up whenever it reads zero to keep feeding the first bacterial colony.
- Weeks 4–6: nitrite falls away, nitrate rises. The finish line is in sight.
A tank with live rock and mature media can be done in a fortnight. A tank of dry rock and dry sand may take six weeks or more, especially in cool weather.
When a cycle stalls
Check the obvious things first. A pH slump caused by alkalinity being used up will slow everything down, so test KH and correct it. Temperatures below about 20°C put the brakes on. Overdosing ammonia above roughly 5 ppm can inhibit the very bacteria you are trying to grow, so do a partial water change and start again at 1 ppm. Poor flow and a lack of surface area on the rock also slow colonisation. And if you are topping up with tap water, stop — use RO water instead.
The fishless cycle checklist
- Confirm salinity, temperature and flow, and that no livestock is present.
- Dose ammonia to 1–2 ppm and record the reading.
- Add bottled bacteria and, if possible, a piece of mature media or live rock.
- Test ammonia and nitrite every day or two at the same time.
- Whenever ammonia reads zero, dose back to 1 ppm to keep the colony fed.
- Wait for nitrite to fall to zero.
- Confirm the cycle: dose to 1 ppm, then retest after 24 hours. Both ammonia and nitrite should read zero.
- Do a 25–50% water change to bring nitrate down, and reset salinity and temperature.
- Add the first livestock, feed lightly, and test every few days for a fortnight.
Ready for the first livestock
A cycle is confirmed, not assumed. The 24-hour test is the one that matters: if the tank can process 1 ppm of ammonia to zero ammonia and zero nitrite within a day, it is ready. If nitrate is very high, the water change is not optional — it is the difference between a clean start and a stubborn algae problem.
Stock slowly. One or two small, hardy fish, or a modest clean-up crew, is plenty for the first fortnight. Then add more over the following weeks, letting the bacteria catch up with each new mouth to feed. Keep an eye on ammonia and nitrite for the first month after the first fish goes in, and feed sparingly. If you are unsure about your water or you keep sensitive species, the staff at a good marine shop are worth asking before you buy.
Photo: arnaud audoin / Pexels
