How to cycle a fish tank: the fishless method, step by step

A new filter holds almost none of the bacteria that break down fish waste. Cycling grows them before any fish go in, by feeding the filter bottled ammonia until it clears a full dose within a day. Without bottled bacteria, a fishless cycle takes 30 to 45 days, about four to six weeks. The work takes a liquid test kit, a bottle of pure ammonia, a dechlorinator and a heater.

The bacteria a new filter lacks

Fish give off ammonia mostly through their gills, and uneaten food rots into more of it. Even a little ammonia damages gills, and more of it kills fish. Ammonia-oxidizing bacteria turn it into nitrite, which is also toxic. Nitrite-oxidizing bacteria then turn nitrite into nitrate, which builds up until a water change takes it out.

Both groups grow very slowly, and the nitrite-oxidizers trail behind. That lag is why nitrite keeps climbing for a week or two after ammonia starts to fall. The bacteria live as a film on the filter media and the top layer of gravel. They need oxygen, so the filter runs the whole time.

A test kit that reads ammonia

Buy a kit that reads ammonia, nitrite, nitrate and pH before the first dose. API's Freshwater Master Test Kit is a liquid kit that covers all four. The same company's 5-in-1 strips read pH, KH, GH, nitrite and nitrate. Ammonia comes on a separate strip.

An Auburn University thesis from 2015 tested API and Tetra strips against lab methods. The strips agreed with the lab's decision on nitrite in 77 percent of samples, and on nitrate in only 41 percent. That makes a liquid kit reading all four the safer buy for a cycle.

Most ammonia kits read total ammonia, the free and ionized forms together, but they report it in different units. Bottled cycling ammonia is labeled as ammonia-nitrogen, so a 2 ppm dose can read about 2.4 to 2.7 on some kits. Landing exactly on 2 does not matter.

Pure ammonia, and how much to add

Ammonium chloride sold for fishless cycling is the simplest source, because its strength is printed on the bottle. DrTim's Ammonium Chloride and Fritz Fishless Fuel both hold 40 milligrams of ammonia-nitrogen per milliliter. Both labels give the same rule of 4 drops per gallon for 2 ppm. Start at 2 to 3 ppm, and never go above 5.

A 20 gallon (75 liter) tank takes 80 drops, or 4 mL at 5 mL per 25 gallons. Count the water rather than the size on the box, since gravel and rock take up room.

For a half or quarter dose, halve or quarter your tank's row. Test the next day, and adjust only if the kit reads well away from 2 ppm.

DrTim's bottles made before November 2016 called for 1 drop per gallon, so follow the label on the bottle you have. Hardware-store ammonia runs 4 to 10 percent, so the table does not fit it. Add that kind a few drops at a time, testing until the kit reads about 2 ppm.

The fishless cycle, step by step

The steps follow the method DrTim's Aquatics publishes, which keeps ammonia and nitrite under 5 ppm from the first dose to the last.

1. Fill the tank and treat the water with a dechlorinator. Start the filter, and set the heater to about 78°F (26°C). Chlorine and chloramine are toxic to the bacteria, so treat every top-up as well.
2. Dose ammonia to 2 ppm from the table, and write down how much you added.
3. Test ammonia and nitrite every day or two, and check that the pH stays above 7.
4. When nitrite first shows up, add half of the day-one dose.
5. Once ammonia falls below 1 ppm, add a quarter dose every few days while nitrite climbs to its spike. Skip the dose whenever nitrite reads above 5 ppm, and change some water to bring it down.
6. When ammonia and nitrite both read below 0.2 ppm, add a full dose again.
7. Test 24 hours later. The tank is cycled when ammonia and nitrite both read zero and nitrate shows on the kit. If either one still shows, wait until both drop below 0.2 ppm and repeat the full dose.
8. Change 50 to 70 percent of the water to bring nitrate down, then add the first fish.

What the test kit shows each week

Ammonia barely moves in the first week, and nitrite reads zero. The first trace of nitrite is the sign that the ammonia-oxidizers are at work. Writing every reading on a log sheet makes each turn in the numbers easy to spot.

Nitrite rises through the second week while ammonia falls, and ammonia usually drops under 1 ppm by days 9 to 12. The nitrite peak follows between days 14 and 20. This is the stretch where a cycle can look stuck, because the nitrite-oxidizers are the slower group.

Timothy Hovanec's 1998 study of aquarium bacteria tracked a new tank whose ammonia was gone by day 12. Its nitrite peaked on day 22 and read zero by day 38, about five and a half weeks in.

Nitrate climbs as nitrite falls, and any nitrate at all is a sign the second group is working. Nitrite drops quickly once it turns, which is the time to start the full-dose tests.

TankBud's cycling profile sets the ammonia and nitrite targets at 0.02 ppm or less. A dosed fishless tank reads above both until the cycle finishes, and that is expected.

pH

6.5 to 8.2

Ammonia

up to 0.02 ppm

Nitrite

up to 0.02 ppm

Nitrate

up to 40 ppm

Temperature

72 to 84°F (22.2 to 28.9°C)

New tanks still establishing the nitrogen cycle. TankBud reads every water test against these targets when a tank uses this profile.

Seeded media, bottled bacteria and a warm tank

Live media from an established filter is by far the best way to seed a new one. Split the old filter's media into two equal portions, and the donor filter's dip in performance stays brief. Media carries the old tank's diseases along with its bacteria, so take it only from a tank you know is healthy.

Bottled live bacteria are sold to pair with the ammonia. They can speed up a fishless cycle, but they cannot be relied on to finish one quickly.

Hovanec's study also found Nitrospira doing the nitrite step in freshwater tanks. Nitrobacter, the genus older books name, was not detected, even in tanks dosed with a Nitrobacter product. Whatever the bottle or the borrowed media holds, the full-dose test still decides when the tank is ready.

Warm water speeds every stage. The bacteria work fastest at 77 to 86°F (25 to 30°C), and cooler water slows them without stopping them. Alkaline water helps as well. A pH of 7.3 to 8.0 suits them best, and they slow as the pH drops below 7.

When a cycle stalls

Check the pH first when ammonia or nitrite stops falling between tests. A pH below 7.0 is the usual cause, because nitrification makes acid. The bacteria also use up the water's KH, so keep at least 4.5 dKH while a tank cycles. TankBud's cycling profile flags a pH under 6.5 for this reason, and nitrification stops entirely below 6.0.

A 20 to 25 percent water change usually gets it moving again, and a KH buffer helps where the tap water is soft.

Too much ammonia or nitrite stalls a cycle as well. Ammonia above 5 ppm can hold nitrification back, and nitrite above 5 ppm starts to poison the nitrite-oxidizers. Change water to bring both under 5 ppm, then go back to quarter doses.

A top-up of untreated tap water can stall a cycle on its own, through the chlorine or chloramine it carries. A conditioner that removes or locks up ammonia works against the dose, so cycle with one that treats only chlorine and chloramine.

Tanks filled with pure RO water can stall at the nitrite step. Nitrite conversion stops when the water holds no phosphate at all. A water change with dechlorinated tap water puts some back.

Adding fish to a cycled tank

Stock the tank a few fish at a time, because the bacteria grow only to match the ammonia you fed them. A medium to heavy load of new fish will probably outrun them and bring a spike in ammonia and nitrite.

Watch ammonia and nitrite for a few days after each group goes in. Small readings should clear within a day or two. Leave some time before the next group. Plan the whole stocking in the stocking calculator first, so each addition moves toward a known total.

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Fish-in cycling

The fish supply the ammonia in a fish-in cycle, so they swim in it until the bacteria catch up. Ammonia and nitrite can climb high enough to kill them, which is why many keepers skip the method.

Warm, alkaline water makes the same ammonia reading more dangerous. A higher pH or temperature shifts more of it into free ammonia, about 100 times as toxic as ammonium. Nitrite can harm fish at as little as 0.1 ppm.

The water changes that protect the fish also slow the cycle. Each change removes food the bacteria need, so the filter takes much longer to mature.

Start with a few small fish and feed them very lightly. Test ammonia and nitrite every day, and change at least a third of the water whenever either one shows on the kit. An ammonia-binding conditioner can carry fish through a spike, but it never replaces the water change.

Bottled bacteria can go in with the fish and help the cycle along. Bottled ammonia never goes into a tank with fish.

Brackish tanks

Brackish tanks cycle on the doses and 24 hour finish line used for fresh water. Fishless Fuel is sold for fresh and salt water alike, and salt water usually takes longer to cycle.

Mix the salt to the specific gravity you plan to keep before the first dose. Researchers in Norway moved freshwater-grown filters into salt water for a 2019 study. Their ammonia capacity fell once salinity passed about 4 to 8 ppt, roughly 1.003 to 1.006. At full seawater the filters lost 50 to 90 percent of it, and it came back only with time.

Retest ammonia and nitrite after any later change in salt. The brackish cycling profile keeps the same targets and adds one for salinity.

pH

6.5 to 8.2

Ammonia

up to 0.02 ppm

Nitrite

up to 0.02 ppm

Nitrate

up to 40 ppm

Temperature

72 to 84°F (22.2 to 28.9°C)

Salinity

1.002 to 1.022

New brackish tanks still establishing the nitrogen cycle. TankBud reads every water test against these targets when a tank uses this profile.