A fishless cycle moves through three stages you can see on your test kit: ammonia falls first, nitrite rises and then falls, and nitrate climbs the whole way. The tank is cycled when you can dose ammonia to 2 ppm and read zero ammonia and zero nitrite 24 hours later. Without a bottled bacteria product that usually takes around a month, sometimes longer in saltwater, and an effective product can cut it down a lot.
How do you set up a fishless cycle? #
You need an ammonia source, a running filter, a heater, and a test kit that reads ammonia, nitrite and nitrate.
- Set up the tank the way it’ll run. Filter on, heater on, substrate and decor in place. Dechlorinate the water.
- Pick a clean ammonia source. Pure ammonium chloride sold for fishless cycling is the easiest. Household ammonia works only if the label lists nothing but ammonia and water, with no surfactants or scent. Fish food works too, but it’s slow and messy.
- Dose to about 2 ppm. Dr Tim’s Aquatics, which sells a cycling ammonia, says 4 drops of its current formula per gallon gives 2 ppm. It suggests counting your water volume about 20% smaller than the tank’s size to allow for rock and substrate. For other products, dose a little, test and adjust.
- Add bottled nitrifying bacteria if you’re using them. Follow that product’s directions. Dr Tim’s says to switch off UV sterilizers, protein skimmers and ozone for 48 hours after adding its bacteria.
- Test the next day, then follow the routine below.
Don’t push ammonia higher to speed things up. The Dr Tim’s fishless cycling guide warns against letting ammonia or nitrite go above 5 ppm, because it doesn’t help and can stall the cycle.
What should fishless cycle readings look like week by week? #
Every tank runs on its own timeline, so treat this as the usual shape of a cycle without bottled bacteria, not a schedule. The pattern matters more than the exact days.
| Stage | Roughly when | Ammonia | Nitrite | Nitrate | What’s happening |
|---|---|---|---|---|---|
| Start | Days 1-7 | 2 ppm, barely dropping | 0 | 0 | Ammonia-oxidizing bacteria are only starting to grow |
| Ammonia drop | Weeks 1-3 | Falling faster after each dose | Rising | 0 to barely visible | Ammonia is being converted to nitrite |
| Nitrite peak | Weeks 2-4 | Clears within a day or two | Often maxes out the color card | Starting to show | Nitrite-oxidizing bacteria lag behind |
| Nitrite fall | Weeks 3-6 | 0 within 24 hours | Falling | Clearly rising | The second group of bacteria catches up |
| Cycled | Often weeks 4-6 | 0 within 24 hours of a 2 ppm dose | 0 within 24 hours | High | Ready for a water change and fish |
Dr Tim’s puts natural cycling at 30 to 45 days without an additive, with saltwater usually taking longer. With an effective bacteria product, it says you may be able to add a full dose and read zero ammonia and nitrite the next day within five to seven days. Hobbyist reports are more mixed, so plan for a few weeks even with bottled bacteria and be pleased if it goes faster.
Why doesn’t nitrate show up for weeks? #
Most hobby nitrate kits read poorly at low levels, and nitrate only builds up once nitrite is being converted. Dr Tim’s notes that most kits won’t show much nitrate for several weeks. Rising nitrate is a good sign when it does appear, since it’s the end product of a working cycle.
What’s a good day-by-day testing routine? #
This follows the routine Dr Tim’s recommends for its product, and it works with any ammonia source.
- Day 1: dose ammonia to 2 ppm, and add bacteria if you’re using them.
- Day 2: test ammonia and nitrite.
- Every day or two after that: test ammonia and nitrite. When both are below 1 ppm, dose back up to 2 ppm.
- When both read below 0.5 ppm on a test day, you’re close. Test daily from here.
- When both are below 0.2 ppm, dose 2 ppm again and test after 24 hours.
- The finish line: 2 ppm of ammonia in, zero ammonia and zero nitrite 24 hours later.
The bacteria don’t need ammonia every day. Dosing on top of ammonia that hasn’t been processed yet is how people push readings past 5 ppm by accident.
Why is my fishless cycle stalled? #
A cycle that stops moving for a week or more usually has one of these causes.
- pH has dropped. Nitrification releases acid and uses up alkalinity, as the University of Florida’s ammonia fact sheet explains. In soft water, pH can fall into the low 6s, and the bacteria slow sharply. Test pH and KH. A water change usually brings both back up.
- Too much ammonia or nitrite. Readings above 5 ppm can slow things down. Do a water change to bring them under that.
- Cold water. The bacteria grow slower in cool water, so keep the heater running at a normal tropical setting.
- Chlorine. Refilling or topping off with untreated tap water kills bacteria. Always dechlorinate.
- Too little surface area. A tiny filter with no media gives the bacteria nowhere to live. Sponge or ceramic media helps.
- The nitrite stage is just slow. The second group of bacteria grows more slowly than the first, and the nitrite peak is the part that tests people’s patience. If ammonia clears daily and pH is fine, keep going.
How do you log a fishless cycle? #
A cycle is easier to read as a chart than as a list. You’re looking for a hill in the ammonia line, a later and taller hill in the nitrite line, and a rising nitrate line under both. When the ammonia and nitrite lines stay on zero the day after a full dose, you’re done.
Fish Tank Tracker logs ammonia, nitrite, nitrate, pH and temperature, with a notes field on each test where you can write “dosed to 2 ppm” or “added bacteria”. Its ammonia and nitrite target bands sit at zero, so you can watch each line come down into range, and a recurring task every few days reminds you to test and dose. The free version holds a month of history, which covers most cycles.
For tips on reading color cards accurately, see how to test aquarium water. If you’re planning to use strips, check that they test ammonia first. Many multi-test strips don’t, as we explain in test strips vs liquid test kits.
When can you add fish after a fishless cycle? #
Once you’ve passed the finish-line test, do a large water change to bring nitrate down, since it will be high after weeks of dosing. Add your first fish that day or the next, before the bacteria run short of ammonia. Start with a few fish rather than the whole stocking list, and keep testing ammonia and nitrite for the first couple of weeks. If either one shows up, our guide to an ammonia spike in a fish tank walks through what to do.
Frequently asked questions #
Can I add fish during a fishless cycle? #
It’s better not to. Cycling with fish in the tank exposes them to ammonia and nitrite for weeks. If fish are already in, keep both readings low with frequent water changes and feed lightly until the cycle finishes.
Does bottled bacteria really speed up cycling? #
Some products can, especially ones containing live nitrifying bacteria that are used before their date. Results vary a lot between products and tanks, so let your test kit be the judge rather than the label.
Why is my nitrite so high and not dropping? #
The bacteria that convert nitrite grow more slowly than the ones that convert ammonia, so a long nitrite peak is normal. If it’s above 5 ppm, do a water change. Check pH too, since low pH slows the process.
Do I need to do water changes during a fishless cycle? #
Only if ammonia or nitrite climbs past about 5 ppm or pH drops too low. Otherwise let the bacteria work, then do a large change at the end before adding fish.