How to Fishless Cycle a Tank Using the 2 ppm Ammonia Method

To fishless cycle a tank, add pure ammonia until your test reads about 2 ppm, then keep feeding that ammonia to the tank while ammonia-oxidizing and nitrite-oxidizing microbes colonize the filter. Test every day or two, top up with smaller doses as the ammonia falls, and keep nitrite from climbing past roughly 5 ppm. The cycle is finished when a full 2 ppm dose disappears to zero ammonia and zero nitrite within 24 hours. Without a bottled starter, expect this to take around a month.

Why cycle without fish

Fish excrete ammonia continuously, and a new filter has almost none of the microbes that convert it. The UF/IFAS extension fact sheet on ammonia in aquatic systems notes that a new biofilter needs six to eight weeks to build enough bacteria to reliably control ammonia and nitrite, and that nitrite is toxic to fish at levels as low as 0.10 mg/L. Supplying ammonia from a bottle instead of from fish lets you push the colony to a useful size without exposing any animal to those spikes. For the underlying biology, see the nitrogen cycle explained.

What you need

Item	Why it matters
Pure ammonia source	Ammonium chloride solution or crystals, or a household ammonia that lists only ammonia and water. No surfactants, scents, or dyes.
Liquid test kits for ammonia, nitrite, nitrate	Strips are hard to read at the low end. A salicylate ammonia kit is less affected by conditioners than a Nessler kit.
pH and KH (alkalinity) tests	Nitrification consumes alkalinity; a falling KH warns you before pH crashes.
Dechlorinator	Chlorine and chloramine harm the bacteria you are trying to grow.
Heater and running filter	Warm, oxygenated water flowing through the media speeds colonization.
Notebook or spreadsheet	Trends matter more than any single reading.

Follow each kit’s instructions exactly, including shaking the reagent bottles and waiting the full color-development time; most misreadings come from rushing these steps.

Before you dose: two details that change the math

Know what your ammonia test reports

Scientific sources and some manufacturers express ammonia as nitrogen (TAN, or NH3-N). Others report total ammonia as NH3/NH4+. The same water gives a number about 1.2 times higher when expressed as NH3 than as nitrogen, so “2 ppm” on one kit is not exactly “2 ppm” on another. For fishless cycling this difference is small enough not to matter; just be consistent and use the same kit for the whole cycle.

Use the real water volume

A “20-gallon” tank rarely holds 20 gallons of water once substrate, rock, and the gap below the rim are accounted for. Subtracting 10 to 15 percent is a reasonable starting estimate. Being a little low on volume simply means your first dose reads slightly high on the test, which is why you always dose, wait an hour for mixing, and test.

How much ammonia to add

The target is roughly 2 mg of ammonia-nitrogen per liter of water. The table below shows what that means in practice. The ammonium chloride figures assume pure crystals (26.2 percent nitrogen by mass); the household ammonia figures assume a 10 percent solution, which is only a guess unless the label states the concentration.

Tank water volume	Nitrogen needed for 2 ppm	Pure ammonium chloride	10% household ammonia	DrTim’s solution (4 drops/gal)
10 gal (37.9 L)	75.7 mg	0.29 g	about 0.96 mL	40 drops
20 gal (75.7 L)	151 mg	0.58 g	about 1.9 mL	80 drops
29 gal (109.8 L)	220 mg	0.84 g	about 2.8 mL	116 drops
40 gal (151.4 L)	303 mg	1.16 g	about 3.8 mL	160 drops
55 gal (208.2 L)	416 mg	1.59 g	about 5.3 mL	220 drops

The arithmetic for a 20-gallon tank: 20 gal × 3.785 L/gal = 75.7 L, and 75.7 L × 2 mg/L = 151 mg of nitrogen. Ammonium chloride is 26.2 percent nitrogen, so 151 ÷ 0.262 = 578 mg, or 0.58 g. DrTim’s ammonium chloride is sold at 40 mg/mL of ammonia-nitrogen with a label dose of 4 drops per gallon; at a typical 0.05 mL per drop that is 8 mg of nitrogen per 3.785 L, or about 2.1 ppm.

Household ammonia is the least predictable option. DrTim’s Aquatics notes that these products typically range from about 4 to 11 percent ammonia, rarely state the concentration, and lose strength as ammonia gas escapes. Add a small measured amount, let it circulate, test, and scale up. The fishless cycle dosing calculator does the conversion for your tank size and product strength.

Step-by-step schedule

Step 1: Set up the tank as if fish were coming tomorrow

Fill, dechlorinate, and run the filter and heater. Warmer water speeds bacterial growth, so many hobbyists hold the tank around 78 to 82°F (26 to 28°C) during the cycle, provided the equipment and any plants tolerate it. Check pH and KH now so you have a baseline.

Step 2: Dose to about 2 ppm and record the amount

Add your calculated dose, wait an hour, and test. Adjust if you are far off, and write down the exact volume or weight you added. That number becomes your “full dose.” DrTim’s guidance targets an initial 2 to 3 ppm and warns against going above 5 ppm.

Step 3: Test every one to three days until nitrite appears

For the first week or so, ammonia may barely move. The first sign of progress is a nitrite reading, which means ammonia oxidizers are working. When you see it, add half of your full dose. Keep testing ammonia and nitrite every couple of days.

Step 4: Feed small doses while nitrite peaks

In the DrTim’s timeline for a cycle without added bacteria, ammonia typically falls below 1 ppm around days 9 to 12, while nitrite keeps climbing and peaks somewhere between days 14 and 20. During this phase, add about a quarter dose every few days to keep the ammonia oxidizers fed, but hold back if nitrite approaches 5 ppm, because high nitrite inhibits the nitrite oxidizers you are waiting on. Once nitrite starts to fall, it usually falls quickly.

Step 5: Run the 24-hour test

When ammonia and nitrite both read zero, add a full dose. Test 24 hours later. If both are zero and nitrate has risen, the filter can process that load. Repeat once more on a following day to be sure it was not a fluke.

Step 6: Change water and add fish promptly

By now nitrate will be high. Do a large water change (or two) to bring it down; see water change math for how to calculate the size. Add fish within a day or two, or keep dosing ammonia daily until they arrive so the colony does not shrink.

A realistic timeline

Phase	What you see	Roughly when (no bottled bacteria)
Lag	Ammonia steady near 2 ppm, no nitrite	First several days
Ammonia oxidizers establish	Ammonia falls, nitrite appears	About days 7 to 12
Nitrite peak	Ammonia low, nitrite high	About days 14 to 20
Nitrite oxidizers catch up	Nitrite drops fast, nitrate rises	Weeks 3 to 5
Cycled	Full dose cleared in 24 hours	Often around day 30, sometimes longer

Treat these as typical, not guaranteed. Cool water, soft water, and very clean new media all slow things down. Adding a few handfuls of media from an established, disease-free filter can shorten the lag considerably, though it carries a small risk of moving pathogens, snails, or other hitchhikers along with the bacteria.

Troubleshooting a stalled cycle

Ammonia or nitrite stopped falling and pH is low. Nitrification releases acid and uses up alkalinity. UF/IFAS notes that common nitrifiers do not work effectively below a total alkalinity of about 20 mg/L, and DrTim’s identifies a pH below 7.0 as the usual cause of a stall. A 20 to 25 percent water change with harder tap water often restarts things; in very soft water, raising KH gradually with baking soda is a common fix. Our GH, KH, and pH guide explains the relationship.

Nitrite is off the chart. Many nitrite kits top out around 5 ppm, so a maximum-color reading may hide a much higher level. Dilute a sample 1:1 with dechlorinated tap water to read it, then do a large water change to get below 5 ppm and cut back on ammonia dosing.

Nothing is happening after two weeks. Confirm the dechlorinator is being used, the filter is running continuously, and the ammonia source is actually pure. Check that your ammonia kit is not expired.

Cloudy or smelly water. This is common with the rotting-shrimp method, not with bottled ammonia. DrTim’s points out that nitrifying bacteria grow too slowly to cloud water, so cloudiness is a different bacterial bloom.

Ammonia reads high after using a “detoxifying” conditioner. Conditioners that bind ammonia can confuse test results, especially with Nessler-type kits. DrTim’s advises against adding ammonia-removing products during a fishless cycle at all.

Common mistakes

• Dosing 4 to 5 ppm or more “to go faster.” It slows the nitrite phase instead.
• Using scented or sudsy cleaning ammonia. If it foams when shaken, do not use it.
• Stopping ammonia doses once the tank is “done” but fish are a week away.
• Declaring success from a single zero reading rather than a full-dose 24-hour test.
• Adding a full stocking list at once. The colony is sized to the ammonia you dosed; add fish in stages and use the stocking planner to keep the load realistic.

What about bottled bacteria?

Products vary a lot. Research on freshwater aquaria found that nitrite oxidation is carried out mainly by Nitrospira-like bacteria rather than the Nitrobacter long named on labels (Hovanec et al., 1998), and that ammonia-oxidizing archaea often dominate established freshwater biofilters while two commercial supplements tested contained ammonia-oxidizing bacteria but no detectable archaea (Sauder et al., 2011). A good product can shorten the cycle considerably, but the proof is the same either way: the 24-hour test with your own kit.