Emergency Fish-In Cycle

Hey there!

Did you just get some new fish and are only just now learning about this cycling thing?


Did the pet store employee tell you that you only need to run your filter for 24 hours before adding your fish?


Maybe you killed all the good bacteria in your tank and have to cycle your aquarium all over again.

Don’t worry! This is all much simpler and more common than it seems.

The best option is to return your fish until you have your tank fully cycled.
If you can’t, then you will have to cycle your aquarium the old-fashioned way – with your fish inside of the tank! 

If you haven’t bought your fish just yet, I really recommend using a fishless cycle, which is an easier, faster and safer way to cycle your aquarium. 

Fish-in cycling is very stressful for your fish. And, many fish won’t survive the process. Those that do are often more susceptible to disease and live shorter lives.

I assume you want to keep your pet fish alive as long as possible, right? Well, a fish-in cycle can be a terrible way to start.

What is fish-in cycling?

The purpose of a fish-in cycle is starting the nitrogen cycle, an invisible three-stage process where good bacteria establishes in your filter, keeping your fish safe by basically eating the ammonia!

Here’s a quick graphic of the nitrogen cycle in action…

Stage 1 Waste such as pee, poop and uneaten fish food breaks down, releasing ammonia, a toxic chemical, into the water.

Stage 2 Beneficial bacteria eat the ammonia and release nitrites, which is also a toxic chemical.

Stage 3 A second type of bacteria eats the nitrites and converts them to nitrates! This is safe in small amounts, this is what we are supposed to remove with regular water changes!

Important: With just a single letter separating nitrites and nitrates, it’s easy to confuse the two. Remember… Nitrites are highly toxic to your fish while nitrates are not harmful in small amounts.

While there definitely are safe ways to do it, most of those who recommend doing a fish in cycle, aren’t taking into account the risks involved. Unless you are using a specific brand bottle, Seachem Prime, there are going to be risks. It’s imperative to your fish’s safety to use Seachem Prime.

You are going to need need 3 very important products to ensure that your tank is cycled correctly!

1. An aquarium test kit

The problem with the nitrogen cycle is that you can’t see it. The only way to monitor the cycling process is to test for it. To do that, you need a good aquarium test kit.

I recommend buying a master test kit like the one pictured above because it includes all the test kits you need to cycle your aquarium at one low price. 

If you’re buying this from a Petco or PetSmart, try to pricematch the price to their online website; the price can probably be lowered at the register.

Strips can be very inaccurate, so it’s highly recommended you stay away from those, and use liquid tests instead.

2. Seachem Prime

Get to know this product. During a fish-in cycle, it’s going to be your best friend. No other water conditioners do what Prime does. 

Seachem Prime serves two purposes. The first is making your tap water safe…

You see, tap water contains chlorine. Not only are these chemicals toxic to fish, but they also kill that beneficial bacteria. And without that, your tank won’t cycle.

Fear not! A dose of Seachem Prime to your tap water before adding it to your aquarium will dechlorinate your water, making it safe for the beneficial bacteria and fish!

Seachem Prime also has the added benefit of neutralizing ammonia, nitrites and nitrates in your aquarium for 24 to 48 hours, making them harmless to your fish while you’re cycling the tank!

This removes the risk of ammonia poisoning while you’re doing your fish-in cycle. I never recommend a fish in cycle without this product.

2. Bacterial Booster

I highly recommend using an additional product from Seachem called “Stability.”

This is basically a bacteria “seed” that gets the right kind of bacteria in the tank quicker! This way, the tank will cycle in 1-2 weeks rather than 3-4 weeks.

With these three products on hand, you are now ready to begin.

How do you perform a successful fish-in cycle?

Before I continue, I want to make one thing clear:

The lives of your fish are at stake!

I cannot guarantee that your fish will survive the cycling process, especially if you’re choosing not to use Seachem Prime. But if you follow this guide, you give them the best possible chance.


Set Up Step 1: Set up your aquarium

It’s time to set up your aquarium. This includes all the equipment – heaters, filters, air pumps…

The beneficial bacteria needs surfaces to cling to, mostly being your substrate and filter media. By setting everything up, you give your bacteria as many surfaces as possible to cling to. You want a substantial amount of substrate, and as many filter sponges and media as you can fit !

You want to keep all electrical equipment, such as heaters, filters and bubblers, switched on throughout the entire cycling process. They should never be turned off.


Set Up Step 2: Putting fish in!

When you add your fish, you really should acclimate them. This will give them the best chance of survival. I recommend floating the bag for 10-30 minutes and then drip acclimating

Be sure to add appropriate doses of Seachem Prime and Seachem Stability. Follow the instructions on the bottle to determine what a single dose would be for your tank – the larger the aquarium, the more Prime you will need.

You also want to feed your fish throughout the cycling process… You don’t want them to starve!

Any ammonia produced by uneaten food will be taken care of by Seachem Prime.

If a fish dies during the cycle, remove it from the aquarium ASAP. Its decaying body will also give off ammonia. Don’t panic if one of your fish dies. It doesn’t necessarily mean that all your fish will – this one might have been extra sensitive.

Now we are ready to begin.


Step 1: Ammonia !

It’s time to use your ammonia test kit every 24 hours.

What you are looking for is any sign of ammonia. Trust me. It won’t be long before it appears.

You will need to dose with prime every 24 to 48 hours. Fortunately, a little goes a long way, and many of you will find that a single bottle of Prime will get you through an entire cycle.

For this method, Seachem Prime is the most important. You see, a dose of Prime will bind ammonia, nitrite and nitrate for 24 to 48 hours, making them harmless to your fish.

Picture Seachem Prime locking up ammonia, nitrite and nitrate in a jail cell for up to 48 hours. While in the cell, these nasties cannot harm your fish. However, once their time is up, and they are released, they will once again attack your fish.

It is for this reason that you need to dose regularly with Seachem Prime, to keep these nasties locked up.

A single dose of Prime will treat up to 1 part per million (ppm) of ammonia. Prime can be dosed up to 5 times safely. So, 2 ppm of ammonia can be treated with two doses of Prime, 3 ppm can be treated with three doses, and so on.

Anything less than 1 ppm should be treated with a single dose. Between 1 ppm and 2 ppm, two doses. You get the idea. Oh, and it goes without saying that it’s better to overdose than underdose. It’s one of the few conditioners that is safe to overdose with.

Once your test kit reads 2 ppm of ammonia, you should perform a 50% water change. This should cut your ammonia levels roughly in half.

Repeat this process until you notice that your ammonia levels are not rising as quickly as before. Typically, this will take a week or two. When this happens, you are ready to move to the next step.

(Seachem Prime is not an ammonia remover, it only neutralizes the ammonia. It does this in a way so that the ammonia is not only safe for the fish, but the bacteria can still use it to grow. It will still show up on a test, even if the fish can’t feel it.)

Step 2: Nitrites

Next, you need to test for nitrites. If none are present, keep repeating the previous step and testing for nitrites daily until you get a positive result.

Now that nitrite has entered the equation, you need to make sure you are dosing with enough Prime to protect your fish from both ammonia and nitrites.

Fortunately, working out your new dosage is pretty simple – just add your nitrite and ammonia readings together.

Let’s say your test returns the following…

Ammonia 0.25 ppm
Nitrite 1.50 ppm

Adding the two together will give you a total of 1.75 ppm. A double dose of Prime will take care of it.

This time, if your combined ppm of ammonia and nitrite reaches 4, perform a 50% water change.

Keep repeating this. You will eventually notice that your ammonia drops to zero. This is a good thing – bacteria are eating the ammonia as quickly as it is being produced, resulting in a zero reading.

Continue daily dosing and testing for ammonia and nitrites. Once your nitrite levels stop rising as quickly, you are ready for the next step.


Step 3: Nitrates

Now, you want to test for nitrates. If none are present, keep repeating the previous step and testing for nitrates daily until you get a positive result.

Once nitrates are present, you are nearing the end (woohoo!). Continue testing for ammonia, nitrites and nitrates. Your ammonia levels should still be zero, and your nitrite levels should be declining.

Keep dosing with Prime until both your nitrite and ammonia levels are zero. At this stage, the beneficial bacteria are eating them as quickly as they are being produced!

Congratulations! You have successfully completed your fish-in cycle!

The only maintenance that will need to be continued are regular water changes weekly or biweekly that will remove any nitrate that builds up! 


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