How to start a new aquarium
A six-week, fishless plan for cycling your first tank — what to buy, what every reading means, and how to know the day it's safe to add fish.
Before the water goes in
Nothing in this guide asks you to hurry, and this part least of all. What you buy this week decides how forgiving the next six weeks are.
Buy a bigger tank than feels sensible. This is the one piece of advice that reliably surprises people, because the small tanks are the ones marketed to beginners. It is backwards. Everything that goes wrong in an aquarium goes wrong as a concentration — ammonia from a dead snail, a heater that sticks on, a fortnight of overfeeding — and a larger volume of water dilutes all of it. Roughly 60 to 120 litres (15 to 30 gallons) is the sweet spot for a first tank: big enough to absorb mistakes, small enough to fit a normal room and a normal budget. A 20-litre desk cube is an expert’s tank wearing a beginner’s price tag.
Where it goes matters more than it looks. Water weighs a kilogram a litre, so a 100-litre tank plus substrate is well over 120 kg standing on four small feet. It needs something genuinely sturdy and genuinely level, and it needs to be out of direct sunlight unless you want an algae bloom for a housewarming gift. Decide the location before you fill it, because a full tank cannot be moved — it has to be drained first.
What you need on day one:
- A filter, rated for your tank size or a little above. This is where most of your bacteria will eventually live.
- A heater, if you are keeping tropical fish — which you probably are.
- A light, on a timer. Six to eight hours a day is plenty and gives algae much less to work with than “whenever we’re home”.
- Dechlorinator, because tap water is treated specifically to kill the bacteria you are about to spend a month growing.
- A liquid test kit. Not strips.
That last one is worth being blunt about. You are about to spend six weeks making decisions based entirely on ammonia, nitrite and nitrate readings, and paper strips are unreliable at exactly the low end where those decisions get made. A liquid kit costs more once and answers the only question that matters for the next month and a half. Buy it now, not after the first confusing result.
What can wait: CO2 injection, fancy filter media, UV sterilisers, automatic feeders, anything described as a “booster”. None of it will speed up what happens next, and some of it will get in the way.
Then set it up and fill it. Rinse the substrate until the water runs clear, lay in your hardscape, fill with dechlorinated water, and switch the filter and heater on. The tank will look finished. It is not — it is just full.
In the app — add the tank with its name, water type and volume, then tap I’ve filled it with water. That moves it into Cycling, stamps a Cycle started entry in the tank’s journal, and schedules a repeating water test every two days, so the testing rhythm the next six weeks depend on is already set up rather than something you have to remember.
Start the cycle
Your tank is full of water and there are no fish in it. That is exactly right, and it needs to stay that way for a few weeks.
Here is what those weeks are for. Fish produce ammonia constantly, and ammonia burns them. In an established aquarium two colonies of bacteria handle it: the first converts ammonia into nitrite, which is also toxic, and the second converts nitrite into nitrate, which is not — at least not at the levels a water change removes. Neither colony exists in your tank yet. Cycling is the process of growing them, in that order, before anything is living in there.
You can grow them with fish in the tank. People do. The fish spend three to six weeks swimming in a low-grade chemical burn, and it is slower besides, because you have to keep diluting the very ammonia the bacteria need. Do it without fish instead.
Give the bacteria something to eat. They need an ammonia source, and you have two reasonable options. Pure household ammonia is the precise one: dose to around 2 ppm and you know exactly what you started with. The label must list ammonia and water and nothing else — no detergents, no perfume, no colour. Shake the bottle; if it foams, put it back. The lazier option is a pinch of fish food left to rot, which works fine and takes longer to read, because you never quite know how much ammonia you added.
Dechlorinate first. Tap water in most places carries chlorine or chloramine specifically to kill the kind of bacteria you are trying to cultivate. Treat the water before you dose it.
Then wait, and test every couple of days.
The first week is usually boring, and this is where most people conclude they have done something wrong. They have not. Bacteria have to arrive and multiply before anything moves, and until then your test results will sit flat and look like failure. Warmth helps — the high twenties Celsius is comfortable for them — and so does a pH above 7. Below about 6.5 the cycle stalls outright, which is worth checking if a week turns into three with nothing happening.
In the app — a tank you have marked as filled reads Awaiting first test until you log one that measures ammonia, nitrite or nitrate; a temperature-only log will not move it, because temperature says nothing about the cycle. Once tests exist but nothing has stirred yet, it reads Cycle starting — a named state rather than an error, because a flat first week is not a fault.
The ammonia phase
Ammonia climbs. This is the goal.
Your first colony has found the food and is multiplying, and the reading going up is the evidence. Nitrite will still be flat, because the second colony has nothing to work with yet. A tank sitting at 2, 3, 4 ppm of ammonia with no nitrite behind it is a tank doing precisely what it should.
The instinct at this point is to fix it. Don’t. A water change now removes the ammonia the bacteria are living on and sets you back several days for no gain. There are no fish in the tank; nothing is being harmed by a number.
On reading the kit. Liquid tests are read against a printed colour card, and the mid-range blocks are genuinely hard to tell apart. Hold the vial against the white part of the card in daylight rather than under a lamp, and read it at the time the instructions say — nitrate in particular keeps developing colour and will read high if you leave it. When two blocks look equally plausible, they probably are. Write down the lower one and move on; the trend across several tests matters far more than any single reading.
This phase commonly runs one to two weeks.
In the app — the stepper moves to Ammonia, and the tank health score stops applying while a tank is cycling. That is deliberate. Scoring a cycling tank the way an established one is scored would flag a perfectly healthy process as a crisis every single day.
The nitrite phase
Nitrite appears. You are past halfway.
Nitrite showing up means colony #1 is working — something is converting your ammonia — and colony #2 has started growing on the results. This is the longer of the two phases, often two to three weeks, and it is where patience tends to run out.
If ammonia and nitrite are both elevated, you are further along than it feels. This is worth sitting with, because the tank looks worse than before: two bad numbers instead of one. It isn’t. Nitrite in the water is proof of conversion that was not happening a week ago. The app ranks a tank this way too — when both readings are up it reports the nitrite phase, not the ammonia phase, because nitrite is the later signal.
When to do a water change. Nitrite can climb until it is off the top of the card, and at that point it starts to inhibit the very bacteria consuming it — the cycle stalls. If your nitrite reading is beyond what the card can show, change half the water, dechlorinate, and carry on. You are not resetting anything; the bacteria live in the filter and the substrate, not in the water you poured out.
Around now, nitrate will start appearing. That is the finish line coming into view.
Leave the filter alone. Do not replace the media, and do not rinse it under the tap — the chlorine that makes tap water safe for you is what you have spent a month cultivating bacteria in spite of. If it genuinely needs cleaning, swish it in a jug of tank water.
Confirming it’s done
Ammonia and nitrite both read zero. It is tempting to call it here. Don’t — one clean reading is the single most common false finish in the hobby.
A clean reading tells you the tank handled the ammonia that was in it at some point before you tested. It does not tell you the colonies are large enough to clear a full day’s load, which is the thing you actually need to know before anything lives in there.
So confirm it deliberately:
- Dose ammonia back to about 2 ppm. Yes, on purpose.
- Wait 24 hours. Do not test early.
- Test again. Ammonia zero, nitrite zero, nitrate present.
A tank that eats 2 ppm of ammonia in a day, leaving no nitrite behind, is cycled. A tank that leaves either behind is close but not finished — wait two days and repeat.
What “zero” means on a hobby kit. It means the lowest block on the card. These kits cannot resolve the difference between nothing and a trace, so anything at or below 0.05 ppm counts as zero, which is the same threshold the app uses. Nitrate at or above 5 ppm is the other half of the test: nitrate is the waste product of the whole chain, and its presence is what proves the conversion ran all the way through rather than the ammonia simply disappearing on you.
One last water change. Six weeks of cycling leaves a lot of nitrate. Change enough water to bring it down before anyone moves in.
In the app — the first clean reading moves the tank to Almost there. It only reports Water is ready once a second qualifying test exists at least 24 hours earlier — the same confirmation you just did by hand, held to automatically, on every test you log.
Your first inhabitants
Ammonia and nitrite are holding at zero, nitrate is present, and you have done the 24-hour confirmation. The tank is genuinely ready. Now do the least intuitive thing in the whole hobby: understock it.
Add a few at a time, and wait two weeks between groups. Your bacterial colonies have grown to exactly the size of the food supply you gave them — the 2 ppm of ammonia you were dosing, and nothing more. A full stocking on day one produces far more waste than those colonies can process, ammonia climbs again, and you get a second cycle with fish in it. That is the same chemical burn you just spent six weeks avoiding, and it is the single most common way a successfully cycled tank still kills its first fish.
A few fish, then two weeks, then a few more. The colonies expand to meet each new load during the gap.
Stock for the tank you actually have. The store’s advice will be optimistic about how much fits, and the internet’s advice is written for someone else’s tank. What matters is your volume, whether you are heated, what your water is like, and — genuinely — what you will enjoy looking at. A species that needs a group of ten is a bad fit for a tank that can hold twelve fish total, however appealing it looked in the shop.
Acclimate slowly. Float the bag to match temperature, then add small amounts of your tank water to the bag over 20 to 30 minutes before netting the fish in. Do not pour store water into your tank.
Keep testing. Weekly is enough now, but test after every new group — a small ammonia or nitrite blip following an addition is the colonies catching up, and seeing it early is the difference between a water change and a loss. Once readings stay flat through a few additions, you are into ordinary maintenance: regular partial water changes to keep nitrate down, and the filter left alone.
In the app — tap Mark as established and the tank graduates: the regular tank health score takes over from the cycle tracker, and the journal records Cycle complete. The every-two-days cycle test relaxes into a normal schedule, and Stocking ideas will suggest inhabitants that suit your volume, your temperature and whoever already lives there — free, and as many times as you like while you make up your mind.
Six weeks ago you filled a glass box with water. What you have now is a working nitrogen cycle that will look after its own chemistry for years, as long as you keep feeding it lightly and changing some water. That is the whole craft. The rest is watching fish.