The single most useful sentence on this site: do not put fish in a new tank. A brand-new filter contains none of the bacteria that make water survivable, and growing them takes about a month. Almost every first-tank failure is that one step, skipped.
This page is the whole first month in order, with links to the page that works each step through in detail. If you are standing in a shop right now, read step one and step two and come back for the rest tonight.
The ten steps, in order
Nothing here is optional and the order matters — each step depends on the one before it.
- Buy the biggest tank your space and budget genuinely allow, with a lid and a light. A 20 gallon tank is easier to keep alive than a 5 gallon one, because every mistake is a concentration problem and more water dilutes it.
- Add the three things almost no kit includes: a liquid test kit that reads ammonia, a full-size bottle of dechlorinator, and substrate. The test kit is not optional — ammonia is invisible and it is what kills the fish.
- Size the filter properly. Multiply tank gallons by 6, then divide by 0.7 to get the rated flow to shop for, because a filter loses roughly 30% of its rating to media and head height in service.
- Size the heater to your coldest room temperature, not to the tank size alone. Roughly 5 watts per gallon for a 10 degree lift; 10 watts per gallon for a 20 degree lift in a cold room.
- Set the tank up, fill it with dechlorinated water, and run the filter and heater for a day to check for leaks and to confirm the temperature holds. Do not add fish.
- Start the cycle. Add an ammonia source and, if you have one, a bottled bacteria starter or media from an established tank. Test ammonia and nitrite every day or two and write the numbers down.
- Wait. Ammonia rises, then falls as nitrite rises, then nitrite falls as nitrate appears. This takes four to six weeks without a starter and often two to three with one. Clear water is not the signal; the test numbers are.
- The cycle is finished when the tank processes an ammonia dose to 0 ppm ammonia and 0 ppm nitrite within 24 hours, with nitrate present. Do a large water change to bring the nitrate down.
- Add a few small hardy fish, not the full stocking. The bacterial colony sizes itself to the waste it is fed, so a large addition outruns it even in a cycled tank.
- Test for a fortnight after each addition, then settle into roughly a 25% weekly water change with dechlorinated water at tank temperature. That is the routine from here on.
Why the waiting is the whole thing
It is worth understanding the mechanism rather than taking it on trust, because the mechanism is what makes the waiting bearable.
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, and its toxicity rises with both pH and temperature, so the warm tank a tropical fish needs is also the tank where a given ammonia reading does the most damage. In an established aquarium, none of this matters, because two populations of bacteria living in the filter media convert ammonia to nitrite and then nitrite to nitrate, which is far less toxic and which you remove with water changes.
Those bacteria are not in the box. They are not in the water, not in the gravel and not in the filter cartridge. They arrive slowly, colonize the filter media over weeks, and only then does the tank work as a system. Adding fish before that is asking an animal to live in an accumulating toxin while you wait — which is exactly what is happening in the tank of every beginner whose fish keep dying for no visible reason.
If you already have fish in an uncycled tank
This is a very common place to arrive, usually a few days after a first trip to the shop, and it is recoverable. Do not tear the tank down. You are now doing a fish-in cycle, which means the same process with the fish acting as the ammonia source and your water changes keeping them alive through it.
- Test ammonia and nitrite daily. You are managing a number, not watching a tank.
- Change water whenever ammonia or nitrite is detectable — 25 to 50%, dechlorinated, matched to tank temperature. This is the entire treatment.
- Feed sparingly. Every uneaten flake becomes ammonia. Fish tolerate light feeding far better than they tolerate ammonia.
- Use a conditioner that detoxifies ammonia to buy time between changes. It does not remove ammonia and it is not a substitute for the water change.
- Add bacteria. A bottled starter, or better, media from an established tank, shortens this considerably.
The full emergency walkthrough is on why do my fish keep dying.
What this costs, honestly
The tank is the small part, and the expensive-looking items are mostly not the ones that matter. A test kit and a bottle of dechlorinator together cost less than a single ornament and are the difference between a working tank and a dead one. The recurring costs after setup are food, dechlorinator, replacement test reagents, filter media if your filter takes cartridges, and the electricity the heater draws — none of them large, and a cartridge-free filter such as a sponge removes the biggest of them entirely.
The thing genuinely worth spending on is the tank itself, because volume buys stability and stability is what forgives a beginner's mistakes. Everything else can be modest. The order to spend in is on best aquarium starter kits.
Read next
The five hubs below are the five steps, and they are in order rather than alphabetical. If you only read one, read cycling — it is the step that decides whether any of the rest matters.
