Signs of a Cycled Aquarium | A Comprehensive Guide

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Alison Page


Signs of a Cycled Aquarium

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What are the signs of a cycled aquarium?

That’s what you need to know if you’ve recently set up a brand-new fish tank and you’re eagerly waiting to add your first fish.

Adding fish to a newly set aquarium too soon is a common newbie error that can result in mass fish kills.

So, keep reading our detailed guide explaining how to know if your new fish tank is fully cycled and if it’s safe to add your fish.

How To Know If Your Aquarium Has Cycled

Adding fish to an aquarium cycle that hasn’t properly cycled will almost certainly result in dead or very sick fish due to ammonia poisoning. So, how do you know that your tank has cycled and it’s safe to add fish?

Test your fish tank water for ammonia, nitrite, and nitrates. If the ammonia and nitrite levels are zero and nitrate levels are less than 30ppm, then the tank is cycled, and you can add a few small fish.

How Long Does It Take For Your Tank To Cycle?

Regardless of whether you used the fish-in or fishless method of cycling your tank, the time it takes for the process to complete is more or less the same. This will offer optimum fish health. 

Generally, a new tank takes four to six weeks to cycle fully.

Throughout that time, you must keep testing the water in the tank every couple of days until the levels are as described above, i.e., zero ammonia levels and nitrite levels and less than 30ppm nitrates.

Excess of any of these will result in toxicity for fish. The fish tank cycle process should be handled carefully. 

Is Cycling Your New Aquarium Really Necessary?

So, what happens if you simply fill your new tank with dechlorinated tap water and add your fish?

The dechlorinated water will be clean and free from harmful chlorine and chloramine, so your fish should be fine, right?

But wait! When you add fish to an uncycled aquarium, the fish immediately begin producing waste.

You feed the fish, the fish produce even more waste, and some of the food falls to the substrate, uneaten.

The fish waste and uneaten food quickly decompose, releasing ammonia into the water as it does so.

In an uncycled fish tank, toxic ammonia levels rise quickly since no beneficial bacteria are present to process the chemical.

Your fish will quickly show ammonia poisoning, and many, if not all, will die. You might also have a diseased fish.

So, you can see that it really is essential to cycling your fish tank properly and completely before you add any fish.

Can You Speed Up The Cycling Process?

If your fish tank is taking too long to cycle, you’ll be pleased to learn there is a way to speed up the process.

During the cycling process, you’re encouraging the growth of various species of bacteria that can convert dangerous ammonia into nitrite and nitrite into nitrate.

That can be done by adding organic material, such as fish food, to the water or putting a few “suicide fish” into the tank to produce waste.

As fish food and organic waste decompose, it produces ammonia. Gradually, colonies of beneficial bacteria begin colonizing the biological filter media inside your filtration system and the tank’s surface, where they feed on the ammonia. The ammonia is processed into nitrite by other bacteria and then converted to nitrates.

As mentioned above, it can take four to six weeks or longer for the bacteria to be present in large enough numbers to process the waste completely. However, there are ways to speed up that process.

Aquarium Bacteria Supplements

The easiest way to quickly increase bacteria numbers in your new tank and speed up the cycling process is by adding an aquarium bacteria supplement to the water.

These products are essentially bottles of liquid containing beneficial bacteria. You simply empty the bottle into your aquarium tank water to add the bacteria to the system.

You will still need to wait a couple of weeks or so for the bacteria to become established in your tank and filter media.

However, that’s still a few weeks less than you’d need to wait if you’re using the traditional cycling methods.

Note: Before using an aquarium bacteria supplement, check that the product is still in date.

If the expiry date of the supplement has been exceeded, it’s likely that the bacteria in the liquid are all dead and will be of no benefit to your tank.

Add Beneficial Bacteria From A Cycled Tank

As you know, beneficial bacteria colonize the biological filter media in your filtration system.

So, suppose you take some filter media from an existing, cycled tank and add it to your new tank.

In that case, you’ll introduce plenty of established bacterial colonies to your setup and instantly accelerate the cycling process.

The main potential issue with this method of kickstarting the cycling process is that you must be absolutely certain there are no diseases, parasites, or harmful bacteria in your other tank.

Otherwise, you could add many problems to your new tank and beneficial bacteria!

Add Plants and Substrate From An Existing Tank

Beneficial bacteria live in your biological filter media but proliferate in large colonies in other areas of your fish tank.

Did you know that all your aquarium decorations can host beneficial bacteria? In addition, plant leaves make excellent surfaces on which bacteria can live.

In fact, the whole surface area provided by your gravel or sand substrate is one huge bacteria repository!

So, provided that your existing tank is healthy and free from disease and parasites, you can accelerate the cycling process by adding a few plants and a handful of the substrate to your new aquarium.

If you decide to add plants to your new setup, remember that you will also need to provide sufficient lighting for the plants each day so they can photosynthesize.

Turn Up The Heat!

The beneficial bacteria will struggle to grow and spread if your tank water is too cold.

So, you can help to speed up the cycling process by adding a heater to your tank. Once the water is warmer, the bacteria will grow more quickly, and the cycling process will accelerate.

Is It Safe To Use Antibacterial Medication In An Uncycled Tank?

Suppose you will speed up the cycling process by adding media from an existing tank. In that case, you might think it’s a good idea to use an antibacterial medication in the new tank to kill off any diseases that might be accidentally imported from the old tank.


Antibacterial medication will kill all the bacteria in your tank, including the beneficial bacteria you’re trying to grow!

Common Rookie Errors That Slow Down The Nitrogen Cycle

Signs of a Cycled Aquarium

A few common newbie errors can slow down the nitrogen cycle. It’s important to be aware of these mistakes, so you don’t make them when cycling your new tank.

Failing To Maintain The pH

Did you know that if the water pH in your fish tank is below 7.0, that can slow down or even stall the nitrogen cycle in your fish tank?

You can prevent that by checking your aquarium water’s pH using a test kit.

If the pH falls below 7.0, performing a 20 percent water change can often be enough to increase the pH to the optimum level.

Adding Too Much Ammonia

One proven way of triggering and “feeding” the cycling process is adding a few drops of pure ammonia to a new fish tank.

Beneficial bacteria need ammonia on which to feed, so surely the more ammonia there is in the water, the better it is for the cycling process, right?

Actually, too much ammonia, i.e., more than 5ppm (parts per million), is toxic to beneficial bacteria.

So, if the ammonia levels exceed that, you can stall the cycle altogether. If you add pure ammonia to your setup, check the water levels to ensure they don’t exceed 5ppm.

The same principle applies if you use the fish-in cycling method or add fish food to an empty tank to create ammonia.

Check the ammonia levels daily throughout the cycling process, and carry out a partial water change of 20 percent to dilute the levels if necessary.

Adding Too Many Fish Too Soon

Once the ammonia and nitrite levels in your fish tank drop to zero and the nitrates are below 30ppm, you’re sure to want to hurry and add new fish to the setup.

However, you must remember that the beneficial bacteria colonies in the tank are very young and recently established.

Although they can process the ammonia produced by a bit of fish food or a few drops of pure ammonia, adding a whole bunch of fish can prove disastrous for your new tank!

Fish begin to produce waste as soon as they hit the water of your new tank. Add to that some leftover food, and you can see how ammonia levels in the water can quickly skyrocket.

If you add too many fish to your new tank, the waste they produce will overload your filtration system, leading to high ammonia and nitrite levels in the water and unhealthy fish.

Start Small

To avoid that problem, start by introducing a couple of small fish first and allowing the beneficial bacteria to adjust to increased ammonia levels.

Be very careful that you don’t overfeed the fish, and be ready to test the water every day for the first couple of weeks to see that the ammonia and nitrite levels stay at zero.

Once the water levels remain correct, you can introduce a few more fish. Continue to monitor the water quality and carry out weekly partial water changes to maintain a clean, healthy environment.


In this part of our guide on identifying the signs of a cycled aquarium, we answer a few of the most commonly asked questions about the subject.

Q: For How Long Should You Cycle a Fish Tank?

A: You should cycle a fish tank until the levels of ammonia and nitrites are zero and nitrates are below 30ppm. That can take from two to six weeks or sometimes longer.

Q: Can a New Tank Be Cycled in Two Weeks?

A: Yes, it is possible to have a tank fully cycled in as little as two weeks. You can shorten the cycling time by accelerating the process using one of the methods described above.

Q: What Do You Do Once Your Fish Tank Is Cycled?

A: Once your new tank is fully cycled, you can add a few small fish. Don’t rush to add too many fish, which could overload your filter system.

Test the aquarium water daily after the fish are added to check that ammonia and nitrite levels remain at zero. Once the environment is stable, you can add a few more fish, continuing to test the water regularly.

Never try to cram too many fish into your fish tank! Overcrowding your aquarium leads to poor water quality, stress, and sick fish.

Q: How Do You Know if Your Tank Is Cycled?

A: Your tank is cycled when the levels of ammonia and nitrites are zero and nitrates are below 30ppm.

Q: How Long Does It Take for Ammonia To Turn Into Nitrite?

A: You should see nitrites in your fish tank water within 11 to 14 days. If that doesn’t happen, check that the water is warm enough and that the pH level is within recommended water parameters.

Final Thoughts

Did you enjoy our guide on identifying the signs of a cycled aquarium? If you did, please share the article before you go!

Basically, a fish tank is said to be cycled when the levels of ammonia and nitrites are zero and nitrates are below 30ppm.

At that stage, you can add a couple of small fish, although you must continue monitoring the levels of toxins in the water to see that the environment is safe for the fish.

You can speed up the cycling process by adding an aquarium bacteria supplement to the water, ensuring that the product is still in-date.

If you have an existing healthy, disease-free tank, you can add a handful of bacteria-rich substrate or a few plants to your new setup to kickstart or accelerate the cycling process.

How long did it take for your new aquarium to be fully cycled? Tell us in the comments box below.

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