How to Cycle a Fish Tank: Nitrogen Cycle Guide

The cycle in one picture

Cycling means growing enough helpful bacteria to deal with the waste your animals make. These bacteria live on the filter material and other surfaces. A tank is “cycled” once they keep up.

What ammonia, nitrite and nitrate are

Test kits report these in ppm (parts per million), which is the same as mg/L.

• Ammonia is the main waste fish give off, mostly through their gills, and rotting food adds more. It damages gills at low levels and kills at higher ones. It has no color or smell, so only a test can find it. A healthy tank reads zero.
• Nitrite is what the first group of bacteria makes from ammonia. It can be toxic from as low as 0.1 ppm. A healthy tank reads zero.
• Nitrate is what the second group makes from nitrite. It is much less toxic, but it builds up over time. Older guidance treated levels up to 200 ppm as harmless, and newer research suggests that was too relaxed. Keeper sources suggest a water change at 40 ppm.

Most ammonia kits read total ammonia. The share in its most toxic form rises with pH and temperature.

Two ways to cycle

We recommend fishless cycling, because no animal is at risk. Sources differ on this: a veterinarian writing for PetMD says the best results often come from cycling with fish in the tank, using a bacteria starter. Cycling with fish is here for anyone who chooses it, or has already bought them.

A veterinary manual warns that cycling by adding fish can push ammonia or nitrite high, and calls the practice potentially inhumane.

If fish are already in the tank

1. Stop adding animals

   More animals mean more ammonia, so wait until the cycle has finished.

2. Feed less

   Sources differ here. A university guide says to reduce feeding or pause it when ammonia shows up, and that a pause of 1 to 3 days does not usually harm fish, except fry (baby fish). A fish veterinarian says to avoid fasting fish to speed up the cycle. Our advice: feed lightly, and remove anything uneaten.

3. Test often

   Test ammonia and nitrite, daily if needed. Test your tap water too, because some tap water contains ammonia.

4. Change water as soon as ammonia shows

   One university guide says any ammonia needs action right away, and that a 25% to 50% water change helps remove it if your tap water has none. A veterinary manual calls 2 ppm of total ammonia high and says to change at least 50% at that point. So 2 ppm is already high, not a level to wait for. A fish veterinarian sets the bar far lower: change water at ammonia above 0.1 ppm or any nitrite, and do not replace more than 50% at once. Treat new water with conditioner.

5. Ask a fish vet if fish look ill

   Lethargy or refusing food can follow ammonia or nitrite poisoning. A vet can advise on treatment.

Fishless cycling step by step

1. Set up the whole tank

   Fill it, add gravel, and run the filter nonstop. Treat the water with a conditioner that removes chlorine and chloramine, or the bacteria will not grow.

2. Check the temperature

   Bacteria grow faster in warm water. One keeper FAQ says cycling is slower below 70°F (21°C). One keeper guide suggests about 82°F (28°C) and warns against going above 86°F (30°C).

3. Add ammonia a little at a time

   Add a few drops, wait a couple of minutes, and test until you reach your target. Sources differ: a veterinary manual says 1 to 5 ppm, one keeper guide says 3 ppm, and another says to hold 1 ppm because high doses can hold the bacteria back. Starting near 1 ppm is the cautious choice. We give no drops per gallon because the amount depends on the strength of your bottle.

4. Test and top up daily

   Test ammonia each day and top it back up to your target.

5. Watch for nitrite

   Nitrite usually does not show in the first week. If it climbs above 3 ppm with no nitrate showing, one keeper guide advises diluting it below 2 ppm with a water change.

6. Wait for nitrate

   As nitrite falls, nitrate should rise. If progress stalls, test pH and alkalinity. Very soft water can stop the bacteria, but sources give different cut-offs, so we give no figure here.

Allow time. Sources give 2 to 6 weeks, 4 to 6 weeks or longer, 4 to 8 weeks, and 6 to 8 weeks for a new filter. One retailer says a few weeks to months.

How to know it is finished

Add your usual dose and test the next day. The cycle has finished when:

• ammonia reads zero
• nitrite reads zero
• nitrate is showing

One keeper guide sets a stricter test: 1 ppm of ammonia gone within 6 hours, then stepping the dose up to 4 ppm the same way. It says this builds a larger colony of bacteria.

What to do once it is finished

1. Keep feeding the bacteria

   Keep dosing ammonia until animals arrive, so the bacteria do not run out of food. Stop before the first animal goes in.

2. Lower the nitrate

   Do water changes if nitrate is above 40 ppm. Test your tap water first, since it may carry nitrate of its own.

3. Choose animals that fit

   Start with our tank-size list, or browse all fish and shrimp and snails. Each fish page lists gear, including a test kit. If you warmed the tank for cycling, bring it to the range on your animal’s species page before adding animals.

4. Add animals gradually

   A cycled tank is sized to the waste it has been handling. Add a few animals, test, and add more only while ammonia and nitrite stay at zero.

5. Keep up the routine

   Test weekly, and any time your fish act differently. Remove old water when you change it, rather than only topping up. Live plants also use up nitrate.

Beginner mistakes

• Rinsing filter media under the tap. Tap water kills the bacteria. Rinse it in old tank water or dechlorinated water.
• Swapping out filter media. The bacteria live there. Leave it in place during the cycle, and if you have two sponges, clean them at different times.
• Adding more fish too soon. This causes new tank syndrome, where ammonia builds up before the filter can cope.
• Overfeeding. Extra food means extra ammonia.
• Guessing instead of testing. A clear tank can still hold toxic ammonia.
• Treating fish without checking the filter. Some medicines, especially antibiotics, can harm the bacteria, so test more often after using one.

Bottled bacteria and borrowed media

Bottled bacteria may speed things up, but sources differ. One retailer that sells it and one keeper guide are positive. A keeper FAQ reports mixed results and says freshness is hard to judge, because the bacteria cannot live indefinitely in a bottle. A fish veterinarian says products claiming to establish the cycle instantly are generally ineffective, and that ammonia binders may slow the bacteria. Whichever you choose, it does not replace testing.

Media from an established tank moves live bacteria straight into your filter. You can use a piece of sponge or filter floss, or run a sponge filter in the other tank first. Keep it wet and move it quickly, since the bacteria need oxygen.

Borrowed media carries a real disease risk. Tanks can pass on parasites, other pathogens and snails, and store tanks are a particular concern. Use a tank you trust that is free of disease. Keep testing afterward.