How to Cycle a Fish Tank: How Long Filter Bacteria Take, the Steps, When to Add Fish, and Fixes for Cloudy Water or Weak Fish

In short: how long does it take to cycle a fish tank?

Sources put it at about 4 to 6 weeks. Decide that it is done by whether ammonia and nitrite have reached 0, not by the number of days.

Cycling a tank means growing bacteria in the filter and the substrate. These bacteria break down the toxins that come from fish waste.

The trouble is that you cannot see how far cycling has gone. Even when the water looks clear, toxins may remain.

This article shows how to follow the progress with water test numbers.

What this article covers:

• The bacteria you grow when cycling, and the nitrogen cycle
• The steps of cycling, and why the timing varies
• The role of bacteria starters and live plants
• How to test the water, and the signs that cycling is done
• When to add fish, and how many
• What to do about cloudy water or weak fish

Cycling a tank goes through four stages. You can tell which stage you are in from the test numbers.

The key point: cycling is done once you have seen nitrite appear and then fall back to 0.

What bacteria do you grow when you cycle a fish tank?

You grow two kinds of bacteria. One turns ammonia into nitrite, and the other turns nitrite into nitrate.

A University of Florida (UF/IFAS) guide calls this process "nitrification." The bacteria that convert ammonia and those that convert nitrite are different.

These bacteria live mostly on the surfaces of the filter media and substrate, rather than in the water. Over-cleaning the filter reduces them.

What you grow when cycling a tank is not the water but the number of bacteria living on the filter media and substrate.

How does nitrogen move through a cycling fish tank?

Nitrogen in a tank changes from ammonia to nitrite, and from nitrite to nitrate. Nitrate is removed by water changes and live plants.

In order, it goes like this.

1. Ammonia comes from fish waste and leftover food
2. The first bacteria turn ammonia into nitrite
3. The second bacteria turn nitrite into nitrate
4. Nitrate goes down through water changes and plant growth

Here is how the three substances differ.

The University of Florida guide says the more fish are fed, the more ammonia they produce. Almost all the nitrogen in a tank comes from food.

Ammonia becomes more toxic as pH and temperature rise. The same reading is more dangerous in warm, alkaline water.

Cycling a tank means getting the bacteria able to carry the nitrogen that starts with food all the way through the cycle.

What are the steps to cycle a fish tank?

Go in this order: place, rinse, fill, run, test. Fish go in last.

1. Place the tank on a level stand that can carry the weight
2. Rinse the substrate and spread it out
3. Fit the filter, heater and thermometer
4. Place plants and driftwood, and fill with water treated with water conditioner
5. Run the filter and heater, and let the temperature settle
6. Add a small amount of an ammonia source to feed the bacteria
7. Test ammonia, nitrite and nitrate every few days
8. When ammonia and nitrite reach 0, change the water and add fish

Tokyo Aqua Garden gives a guide for substrate depth: about 1 to 3 cm without plants, and about 5 cm if you plant.

Tap water contains leftover chlorine from disinfection. This is because Japan's Waterworks Act requires chlorine to remain in water at the tap.

In fishless cycling, you add an ammonia source as food for the bacteria. Practical Fishkeeping's method is to measure and add ammonium chloride.

Steps 6 and 7, feeding the bacteria and testing, are the heart of cycling a tank.

How long does it take to cycle a fish tank?

Sources range from about 2 weeks to 8 weeks. For home aquariums, 4 to 6 weeks is the figure most often given.

Here is the timing each source gives.

The timing varies because conditions differ. According to Practical Fishkeeping, it depends on temperature, filter, water chemistry and filter media.

The same source also lists situations where cycling stalls.

• Below pH 6, the bacteria tend to stop working
• When carbonate hardness (KH) drops, cycling stalls
• If it stalls, make a large water change and start adding food for the bacteria again

Count the time it takes to cycle a tank not by the calendar, but up to the day nitrite falls back to 0.

Do you need a bacteria starter to cycle a fish tank?

A bacteria starter can help cycling go faster. But it does not guarantee the tank is cycled, so test to make sure.

Practical Fishkeeping says bacteria starters on their own are unreliable. If you rely on them too much, you may end up cycling with fish in the tank.

Here are the ways to move cycling along.

Moving filter media is the fastest method. The same source warns not to take too much from the original tank.

Whether or not you use a bacteria starter, the sign that cycling is done is the same: ammonia and nitrite at 0.

Do live plants help cycle a fish tank?

Live plants use nitrate and other compounds as nutrients, so they help when cycling. But they do not replace the bacteria.

According to Practical Fishkeeping, nitrate goes down through water changes and plant growth. The more plants there are, the less nitrate builds up.

Seriously Fish writes this about otocinclus: they have fewer problems when added to a settled tank with plenty of plants.

Points to watch with live plants:

Plants have the clearest role when you add them to help keep the water stable after cycling is done.

How do you test the water when cycling a fish tank?

Test ammonia, nitrite, nitrate and pH. Test every few days while cycling, and about once a week after it is done.

There are two main kinds of test.

The University of Florida guide also touches on types of ammonia test. With some water conditioners, one type of test reads too high.

Test the water before a water change. After the change, you cannot tell the real reading for that day.

While cycling, it is important to keep testing every few days, even if only for ammonia and nitrite.

How do you know when a fish tank is cycled?

When ammonia and nitrite are both at 0. Without fish, a good sign is that added ammonia drops to 0 within a day.

Here is how Practical Fishkeeping checks a fishless cycle.

Nitrite appearing first, followed by nitrate, is a sign that cycling is progressing.

If nitrate is present and ammonia and nitrite are at 0, the tank is cycled.

When and how many fish can you add after cycling a fish tank?

Once cycling is done, start with a small number of fish. Do not add them all at once; add them in stages.

Practical Fishkeeping recommends adding fish gradually. The bacteria grow to match the amount of waste at the time.

Here is the order to add fish, and what to watch afterwards.

A University of Florida guide recommends watching new fish in a separate tank for at least 30 days. This keeps ich and other diseases out.

The steps for moving fish from the bag to the tank are covered in detail in Acclimation.

Choose two species to see the compatibility of your first fish, with the reason.

How to choose fish for beginners is covered in Tropical Fish for Beginners. Each time you add fish, test to check the readings stay at 0.

How do you cycle a fish tank with pilot fish?

Using pilot fish means cycling the tank with fish already in it. It is more work and harder on the fish, so test every day and dilute the toxins with water changes.

Practical Fishkeeping says this method adds work and risk.

The same source also mentions products that reduce ammonia toxicity. They help, but they are not a substitute for water changes.

If you cycle with fish in the tank, keep the number as small as possible, and keep up daily testing and water changes to the end.

What should you do if the water turns cloudy while cycling a fish tank?

White cloudiness during cycling is usually a temporary bacterial bloom. It often clears in 1 to 2 weeks, and repeated water changes can make it last longer.

Aquarium Co-Op explains how to recognize white cloudiness. Scooped into a white cup, it looks like diluted milk, with almost no visible particles.

Here is how to tell the types of cloudiness apart, and what to do.

Algae, which often appear during cycling, are covered in Aquarium Algae.

Practical Fishkeeping says most white cloudiness clears in a few days. Over-cleaning the filter can also set it off.

Keep testing ammonia and nitrite while you wait for the water to clear. If fish are in the tank and the readings are high, do a water change before worrying about the cloudiness.

It is fine to leave white cloudiness alone only when there are no fish, or ammonia and nitrite are low.

Why do fish weaken or die while a fish tank is cycling?

In most cases, ammonia or nitrite has built up faster than the bacteria can break it down. Change part of the water, stop feeding and test the water.

The University of Florida guide says nitrite is toxic to fish even at very low levels. In a healthy tank, ammonia is always 0.

Here are the likely causes and what to do for each sign.

How to recognize and treat ich is covered in Ich (White Spot Disease).

The same guide says that stopping feeding for 1 to 3 days usually does no harm, except to fry. The condition is that the replacement water contains no ammonia.

If possible, watch weak fish quietly in a separate tank. The cause is usually in the water, so putting the water right first is what also protects the other fish.

What are common mistakes when cycling a fish tank?

The three common mistakes are deciding it is done by days alone, adding all the fish at once and over-cleaning the filter.

Most cycling mistakes come from deciding without testing.

What terms come up when cycling a fish tank?

What else should you read along with cycling a fish tank?

Cycling a tank ties in with choosing fish and tank mate compatibility. Pick the article to read next.

• First fish, equipment and costs: Tropical Fish for Beginners
• Compatibility for 23 species at once: Tropical Fish Tank Mates Chart
• If you add shrimp: Shrimp Tank Mates
• If you add bottom fish: Corydoras Tank Mates

What do people often ask about cycling a fish tank?

How long does it take to cycle a fish tank?

Sources put it at about 4 to 6 weeks. Decide by whether ammonia and nitrite have reached 0, not by the number of days.

Temperature and method can make it faster or slower.

If I add a bacteria starter, can I add fish right away?

Not necessarily. A bacteria starter helps speed things up, but the sign that cycling is done is the test numbers.

Add fish once ammonia and nitrite have reached 0.

Do you change the water while cycling a fish tank?

Without fish, as a rule you do not change it; you test. With fish, change at least 1/3 when ammonia or nitrite reaches 0.1 ppm.

Before adding fish, make a large water change to dilute nitrate.

Can I leave cloudy white water alone?

Yes, if there are no fish, or ammonia and nitrite are low. A bacterial bloom often clears in 1 to 2 weeks.

If there are fish and the readings are high, do a water change first.

Should I add live plants when cycling a fish tank?

Adding them helps, because they use nitrate. But they do not replace the bacteria.

Remove dead leaves before they rot.

Can I fill the tank straight from the tap?

Use water conditioner before adding the water. Tap water contains leftover chlorine from disinfection.

Use water conditioner every time for water changes too.

Sources

• UF/IFAS EDIS FA031: Ammonia in Aquatic Systems (where ammonia comes from, two steps of nitrification, oxygen and alkalinity, toxicity rises with pH and temperature, nitrite is toxic even at low levels, 6 to 8 weeks for biological filtration to establish, stop feeding, change 25 to 50% of the water, types of test)
• UF/IFAS EDIS FA006: Ichthyophthirius multifiliis (White Spot) Infections in Fish (watch new fish for at least 30 days)
• UF/IFAS EDIS FA124: The Ornamental Fish Trade (grow bacteria before adding fish, acclimation)
• Practical Fishkeeping: How to Cycle a Fish Tank (timing, fishless cycling and ammonium chloride, the 24-hour sign, pH 6 and KH, bacteria starters, moving filter media, cycling with fish and 0.1 ppm, add fish in stages)
• Practical Fishkeeping: Why won't this cloudy water clear? (white cloudiness often clears in a few days, over-cleaning the filter)
• Aquarium Co-Op: How to Clear Up a Cloudy Fish Tank (recognizing white cloudiness, 1 to 2 weeks, repeated water changes make it last longer, substrate dust, green water)
• Seriously Fish: Otocinclus macrospilus (fewer problems in a settled, well-planted tank)
• Seriously Fish: Corydoras panda (replace leaf litter before it rots)
• Tokyo Aqua Garden: How to keep tropical fish (熱帯魚の飼い方, in Japanese. Substrate depth, run without fish for at least 1 week, about a month to settle)
• Komeri: Let's keep tropical fish in a tank 60 cm or smaller (60cm以下の水槽で「熱帯魚」を飼ってみよう, in Japanese. About 2 weeks of preparation before adding fish)
• Kanagawa Prefecture: Reducing and leveling residual chlorine concentration (残留塩素濃度の低減化及び平準化, in Japanese. The Waterworks Act requires chlorine to remain in tap water)