Cycling a new tank

What is actually happening

Fish excrete ammonia, mostly through their gills rather than as solid waste. Uneaten food and decaying plant matter break down into ammonia too. Ammonia is acutely toxic: it burns gill tissue, and at the pH most tanks run, even a fraction of a part per million causes damage.

In an established tank you never see it, because two groups of bacteria live in the filter and on every surface. The first converts ammonia to nitrite. The second converts nitrite to nitrate. Both live on ammonia the way you live on dinner — no ammonia, no bacteria.

Nitrite is the step people underestimate. It is at least as dangerous as ammonia and it works by a nastier mechanism: it binds to haemoglobin and blocks it from carrying oxygen, so the fish suffocates in fully oxygenated water. Nitrate, the end product, is comparatively harmless at normal levels, and it is the thing your weekly water change exists to remove.

Cycling a tank means growing those two bacterial colonies to the size your intended stock requires, before the stock arrives. Everything else is detail.

Why fish-in cycling is a bad idea

The old approach was to put a few hardy fish in a new tank and let them generate the ammonia. It works, in the sense that the tank eventually cycles. It works by poisoning the fish for several weeks.

Even where the fish survive — and shops recommending "starter fish" rarely mention how often they do not — sub-lethal ammonia and nitrite exposure causes permanent gill scarring, suppresses the immune system and shortens lifespan. A fish that made it through a fish-in cycle is very often the one that dies of something unrelated four months later.

It is also slower and more work: you cannot let ammonia accumulate to the level that grows bacteria fastest, so you spend six weeks doing daily water changes to keep the fish alive while simultaneously removing the ammonia the bacteria need.

The fishless cycle, step by step

1. Set the tank up completely. Substrate, hardscape, filter running, heater set to your target temperature. Bacteria grow considerably faster warm than cold, so if your eventual stock wants 78°F, run the cycle at 80–82°F and bring it down at the end.
2. Dechlorinate. Chlorine and chloramine are put in tap water specifically to kill bacteria, and they are extremely good at it. Every drop of water that goes into the tank from now on gets treated.
3. Get a liquid test kit. Ammonia, nitrite, nitrate and pH. Paper strips are not accurate enough for this job, and they age badly in the tube. This is the one purchase that is not optional — without test results you are guessing, and the whole process is invisible.
4. Dose ammonia to about 2 ppm. Pure household ammonia with no surfactants, dyes or perfumes — it should foam only briefly when shaken. Add a little, test, and work up. Aiming at 2 ppm rather than 4 grows the colony you need without stalling the process, which very high doses can do.
5. Wait, and test every couple of days. Ammonia will start falling and nitrite will appear. This first stage typically takes one to two weeks.
6. Watch for nitrite to spike, then fall. The nitrite-eating bacteria are slower to establish, and this is the stage where people give up. Nitrite can read off the top of the chart for a week or more. Keep dosing ammonia back to 2 ppm each time it drops to zero — if you stop feeding the colony it starves and you lose ground.
7. Test for the finish. The tank is cycled when a 2 ppm dose of ammonia is converted to zero ammonia and zero nitrite within 24 hours. Not "nearly zero", and not once — do it twice on consecutive days to be sure. Nitrate will be high by now, which is exactly what you want to see: it is the proof the chain completed.
8. Large water change, then stock. Bring the nitrate down with a big change, match the temperature, and add your first batch of fish the same day or the next. The bacteria have no ammonia source now, so do not cycle the tank and then leave it empty for a fortnight.

How long it takes, honestly

Three to six weeks is normal. Two is fast. Eight is not unusual in a cold room or with a low pH.

Two things speed it up genuinely. Seeded media — a used sponge, a handful of established substrate or some filter floss from a healthy mature tank, ideally from a friend rather than a shop — can cut it to under a fortnight, because you are transplanting the colony rather than growing it. And warmth, which roughly doubles bacterial growth rate across the range that matters.

Bottled bacteria products vary enormously. Some contain the right species alive and genuinely shorten the process; others contain the wrong species, or the right ones dead after a summer on a warehouse shelf. They are worth trying and they are not worth trusting: keep testing either way, and never treat "I added the bottle" as evidence the tank is cycled.

Things that stall a cycle

• Dosing too much ammonia. Above about 5 ppm, ammonia starts inhibiting the very bacteria you are cultivating. If a cycle sits still for two weeks with high ammonia, do a large water change and restart at 2 ppm.
• pH crashing. The cycle consumes carbonate hardness, and if KH runs out the pH falls through the floor. Below about pH 6.5 nitrification slows down; below 6 it can stop entirely. If your pH is dropping, a water change or a small amount of crushed coral will restore the buffer.
• Rinsing media under the tap. Chlorinated water kills the colony you have grown. Rinse filter media in old tank water, always, for the life of the tank.
• Treating the water with medication or a chlorine remover that binds ammonia. Some conditioners lock ammonia into a form fish tolerate but bacteria still consume — fine — while others and most antibacterial medications will set you back a week.
• No ammonia source. A tank with a filter running in clean water is not cycling. It is just wet.

After the cycle: the first six months

A cycled tank is not the same thing as a mature one. The bacterial colony is sized to the ammonia you have been dosing, biofilm is thin, and there is very little buffer against a mistake. Which is why stocking order matters, and why some species should wait.

Add fish in batches, not all at once — the colony grows to match its food supply, and doubling the load in an afternoon outruns it. Test ammonia and nitrite for 48 hours after each new batch; a small ammonia reading after adding six fish is the tank telling you to slow down, and it is far easier to act on before anything is sick.

Some species genuinely need a mature tank rather than merely a cycled one. Otocinclus need real algae and biofilm to graze and will starve in a spotless new setup. Shrimp need stable parameters more than they need any particular parameter. German blue rams and cardinal tetras are notoriously intolerant of the swings a young tank produces. Those all go in last, months in — which is exactly what the stocking calculator works out for you from your own list.