Signs Of A Cycled Aquarium. How Do I Know If My Fish Tank Is Cycled?

Are you new to the aquarium world and not sure where to start? You might’ve heard people say that you need to have your aquarium cycled. But what does that mean? How are you supposed to know when it’s safe to add fish to your tank?

Luckily, signs of a cycled aquarium are easy to spot. All you need is a good water testing kit and to know what to look for.

What Is The Nitrogen Cycle?

The nitrogen cycle is the process that every new aquarium will go through. During the time that the aquarium is cycling, the water is toxic and dangerous for fish.

So, we don’t recommend adding fish to your new aquarium until your tank has completed its full cycle.

The Nitrogen Cycle In A Nutshell

In short, the nitrogen cycle starts with ammonia. Waste will break down to become ammonia. The ammonia will break down to become nitrite. Then, the nitrites will break down to become nitrate. The nitrates must then come out of the water, usually manually.

In an established aquarium, small amounts of waste aren’t a big deal. Beneficial bacteria will have built up inside your filter media and gravel. Beneficial bacteria break down waste. It keeps the water safe between water changes.

In a new aquarium, beneficial bacteria haven’t had a chance to establish themselves. So, there is no way for the waste to convert into a harmless substance. This leaves waste to build up to toxic levels which can kill your fish.

Keep reading to dive further in-depth into the nitrogen cycle.

How Do I Know When My Tank Is Finished Cycling?

Nobody wants to hear that they have to wait for their aquarium to cycle before they can add fish. An aquarium usually won’t finish establishing for about three months. But, you can usually add fish after about six weeks, albeit carefully.

Luckily, it’s easy to track the nitrogen cycle. This means that you can tell exactly when it’s safe for you to add new fish to your aquarium.


The nitrogen cycle always starts with ammonia. Ammonia is a highly toxic substance that is a result of waste.

In an established aquarium, beneficial nitrogen-fixing bacteria will oxidize ammonia. Oxidation will break the ammonia down into nitrites.

But, if your tank is not established, there will be no bacteria to break down the ammonia. This leads to ammonia build-up. It doesn’t take a lot of ammonia to kill a fish. This is why we don’t recommend adding fish to an aquarium before the nitrogen cycle has finished.

Still, in a new aquarium, you’ll need to jump-start the nitrogen cycle. You can do this by adding a bit of fish food to the aquarium. Don’t worry that you don’t have any fish yet — that’s the point. Without any fish to eat the food, it will begin to break down and convert to ammonia.

You should be monitoring your aquarium’s water parameters every day. The API Master Test Kit is great for this.

You will notice that ammonia levels continue to rise every day. Eventually, your aquarium will begin to build some beneficial bacteria. That’s when you’ll begin to notice your ammonia levels drop and your nitrite levels rise.


Like ammonia, nitrites are incredibly toxic. It doesn’t take high levels of nitrites to kill a fish. In fact, your fish is more likely to die from nitrite poisoning than from ammonia poisoning. Nitrites occur when nitrifying bacteria partially oxidize ammonia.

You should continue to watch your water parameters every day. You will notice that nitrite levels continue to rise every day. As your nitrite levels rise, your ammonia levels will drop.

Eventually, bacteria will build up that like to break down nitrites. That’s when you’ll notice your nitrite levels begin to drop.


Nitrates mark the final step of the nitrogen cycle. It is the final product in the oxidation of nitrogen compounds. Nitrates are significantly less toxic than ammonia and nitrites. Still, they can kill fish if too much builds up in the tank.

At this point in the cycle, your ammonia levels should be down to 0 parts per million (ppm). As your nitrate levels rise, your nitrite levels should begin to drop. Once your nitrites drop to 0ppm, your nitrates should begin to drop as well.

You should allow the nitrate levels to drop as low as possible before adding fish.

What Should The Water Parameters Look Like Before Adding Fish?

As we mentioned before, you should watch your water parameters every day. You’ll know that your aquarium is ready when the water parameters have reached these levels:

While it’s safe to have some nitrates in your tank, no amount of ammonia or nitrites is ever safe.

Does Algae Mean My Tank Is Cycled?

It’s very common to notice algae growth in new aquariums. This leads people to believe that the presence of algae means that their tank is cycled.

But, I’m sorry to say, it’s not as easy as that. Algae is a frequent occurrence in new aquariums, but it doesn’t mean your aquarium is cycled. To know whether your aquarium has finished cycling, you need to test the water.

Can You Speed Up The Nitrogen Cycle?

It’s possible to speed up the nitrogen cycle a bit. Still, it’s a slow process, and you need to be patient. You’ll never be able to speed it up fast enough to add fish in one or two weeks, for example.

Buy Beneficial Bacteria

Yet, you can add beneficial bacteria to your aquarium like Seachem Stability. This product is great because you don’t have to wait for the tank to build up good bacteria by itself.

Check The Price Here

You can add the product to the water and the bacteria will begin breaking down the ammonia right away. It will generally speed up the cycling process by one or two weeks.

This is especially great if you’ve added fish before realizing you shouldn’t have. Your fish will be more likely to survive. The ammonia and nitrite levels shouldn’t climb as high as they would have otherwise.

Use Bacteria From Another Aquarium

You can also use bacteria from other tanks. This method is basically the same as using a bottled product, but it could save you some money.

If you have a friend with a fish tank, ask them to give you their old filter media or a bit of gravel. These items will already contain beneficial bacteria. So, you can add them to your tank to jumpstart the nitrogen cycle.


When we talk about cycling a tank, we’re referring to the Nitrogen Cycle. The tank needs to produce ammonia. Then, it needs to convert that ammonia into nitrites. Lastly, it needs to convert the nitrites into nitrates.

With a good testing kit, you can watch the nitrogen cycle unfold in real-time. Once you learn what numbers to look out for, it’s easy to tell exactly when to add your new fish. How exciting!

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