Tank log and cycle tracker
Log your water tests and see which stage of the nitrogen cycle your tank is actually in, what to expect next, and when it is genuinely safe to add fish.
What this tracks
A new aquarium cannot keep fish alive until bacteria have colonised the filter. Those bacteria convert ammonia (from waste and food) into nitrite, and then a second group converts nitrite into far less harmful nitrate, which water changes remove. Both ammonia and nitrite are toxic, and nitrite is the more dangerous of the two.
The hard part for a first tank is not the chemistry, it is not knowing whether what you are seeing is normal. A nitrite reading of 2 ppm in week three is exactly right. The same reading in week nine means something is wrong. This log works that out from your readings rather than from the calendar, so a tank that cycles in twelve days is called cycled, and a tank still spiking at day fifty is not.
Reading the stages
Ammonia stage. Ammonia is present, nitrite is still at zero. The first bacteria group is establishing. Nitrite appearing is the milestone to wait for, usually around day 10 to 14.
Nitrite spike. Nitrite climbs, often high, while ammonia falls to zero. This is the longest phase and the one that makes people think the tank is broken. It is not. Nitrate climbing at the same time is the proof it is working.
Cycled. Ammonia 0, nitrite 0, nitrate present. Do a water change to bring the nitrate down, then add fish a few at a time so the colony can scale with the load.
If all three read zero for more than a week, the cycle has not started. That almost always means there is no ammonia source, which is the single most common reason a cycle appears to take forever.
If there are already fish in the tank
Set the tank's status to "cycling with fish already in" and test daily. The fish are living inside the cycle, so the goal shifts from waiting to damage control: change water to keep ammonia under 0.5 ppm and nitrite under 0.5 ppm, feed lightly, and use a conditioner that detoxifies both between changes. A fish-in cycle is harder on the fish and takes about as long.
Gear for a 20-gallon tank
- Filtration: aim for a filter rated around four times your volume per hour, so roughly 80 GPH at 20 gallons. See our aquarium filter picks.
- Heating: the standard guideline is about 3 to 5 watts per gallon, scaled by how far above room temperature you are heating. Get the exact figure from the heater size calculator, then see our tested heater picks.
- Lighting: planted tanks need more output than fish-only setups, and a timer matters more than wattage for keeping algae down. See our aquarium LED guide.
- Substrate: choose for your livestock, because corydoras and other bottom-dwellers need smooth sand or fine gravel rather than sharp stone. See our substrate guide.
- Starting from scratch: at this size a kit is usually cheaper than buying the tank, hood, light and filter separately. See our starter kit picks.
Our gear guides contain affiliate links, see our disclosure. They never change our stocking advice or calculator results.
Frequently Asked Questions
How long does it take to cycle an aquarium?
A fishless cycle usually takes four to eight weeks at 78 to 80F. Warmer water (82 to 86F), a seeded filter from an established tank, or a bottled bacteria starter can cut that to two or three weeks.
How do I know when my tank is cycled?
Ammonia 0 ppm, nitrite 0 ppm, and nitrate present. Confirm it by dosing ammonia to about 2 ppm and testing 24 hours later, a cycled filter clears both completely in that time.
Do I need an account?
No. The log is stored in your own browser and never sent to us. That also means it is not a backup, clearing your browser data clears the log, and it will not follow you to another device.
Which test kit should I use?
A liquid test kit, not strips. Strips are cheaper but badly inaccurate at the low readings that matter most during a cycle. The inputs here match the API Freshwater Master Test Kit colour card because it is the one most beginners own.