The steps in pictures
Feed the bacteria. Right looks like this: Fishless: a measured dose of ammonia keeps feeding the bacteria while the filter runs.
Test every day. Right looks like this: Ammonia and nitrite tested daily and written down, with the date.
Ready reading. Right looks like this: Ammonia and nitrite at 0 ppm while you keep dosing; nitrate showing is a good sign, not the pass mark.
Give it weeks. Right looks like this: Three to eight weeks for a new system, longer in cool water; if either reads above zero, wait.
What cycling is
Cycling grows the bacteria that turn fish waste into less harmful forms: ammonia becomes nitrite, and nitrite becomes nitrate. A test kit lets you watch it happen. In a typical cycle ammonia rises and falls first, nitrite climbs as ammonia falls and then drops too, and nitrate builds up at the end, though a planted tank or one started with media from an established filter may never show clear peaks. UF/IFAS, the University of Florida’s extension service, has new systems tested daily for ammonia, nitrite, pH, temperature and alkalinity while they start up, and expects three to eight weeks or more, slower in cooler water.
A fishless cycle feeds the bacteria plain ammonia instead of fish waste; UF/IFAS describes low levels of liquid ammonia with no additives, added before any fish go in. Nitrate is the end product, but UF/IFAS also notes that plants take nitrate up as fertilizer, so a planted tank can be fully cycled and still read little nitrate. A tank a few weeks old with clear water may still not be cycled; only the readings tell you. Check what each reading means as you go.
Which readings, on which days?
- The day you start: test your tap water and the tank, and write down ammonia, nitrite, nitrate, pH and temperature. Zeros in new water only show the water is clean, not that the filter is ready.
- Every day while it cycles: test ammonia and nitrite, and nitrate every few days. In a fishless cycle, top the ammonia back up to about 2 ppm whenever it has dropped, as the ammonia page explains.
- When ammonia and nitrite both read 0: the bacteria are keeping up with what you feed them. Rising nitrate is a good sign, but tap water may already carry some and plants use it, so it is not the pass mark.
- The day before the fish: add the usual ammonia; if ammonia and nitrite read 0 the next day, the filter is ready for the load it has been fed. A big water change then brings the nitrate down before the fish go in.
There is no set number of days: UF/IFAS expects three to eight weeks or more, slower in cool water. Add fish a few at a time and keep testing for the first weeks. A rise in ammonia or nitrite means the bacteria need time to catch up with the bigger load: feed less and change some water until it reads 0 again.
What you need
- The filter running the whole time, set up as its manual says: the bacteria live in it and need water flowing through.
- A liquid test kit for ammonia, nitrite and nitrate. The API Freshwater Master Test Kit covers those and pH; it has no GH or KH test, which you may want for shrimp or soft-water fish. What each kit measures
- For a fishless cycle, pure liquid ammonia with no soap or perfume added.
- A thermometer, and the heater set to the temperature your fish will need: the bacteria grow more slowly in cool water.
- Water conditioner for every water change.
Keep a cycling log
The log at the top of this page keeps your readings in this browser, draws them day by day and tells you where the cycle is. Next to the numbers, note anything that changed that day; a rise you can’t explain usually lines up with one of them. UF/IFAS recommends keeping your results and watching the trend rather than any single test.
| Write down | Why |
|---|---|
| The date | So you see the trend, not one test |
| Ammonia, nitrite and nitrate, in ppm | Each tells you a different stage of the cycle |
| The kit you used | Strips and liquid kits read differently |
| Temperature and pH | Ammonia is more toxic in warm, alkaline water |
| Water changes, filter cleaning, doses, new plants or fish | They explain a sudden drop or rise |
What if the readings are not ready?
| What you see | What to do |
|---|---|
| Ammonia or nitrite still above 0 | Keep testing and don’t add fish yet. Check the filter is running, the surface is moving, and the pH and KH: with KH at 50 ppm (2.8 dKH) or less, Merck notes, the bacteria slow down and the pH can crash. |
| Everything reads 0, but nothing has been feeding the cycle | The zeros only show there is nothing to process; they say nothing about the bacteria. Start adding ammonia or food. |
| Nitrate is up, but so is ammonia or nitrite | Part of the cycle is working. Give it more time. |
| Readings rise again after cleaning the filter, a power cut or new fish | Something set the bacteria back. Hold off on more fish, check the filter is running and feed less until it reads 0 again. Rinse filter media in tank water, not tap water. |
With fish already in the tank, never add ammonia. If ammonia shows, UF/IFAS advises feeding less or stopping for a while and changing some water; fry are the exception, as they need food often. Test and condition the new water too. Fish in distress, or water that won’t clear, need an aquatic vet: take your log.