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Emergency fish in cycling – The safest way

emergency fish in cycling fish in cycle

Many aquarist find themselves doing a Fish in cycle at some point. In many cases this is caused by poor advice from aquarium shops, I’ll name no names ‘Cough pets at home, cough cough’.
So, it’s handy to know the safest way to do it so that your fishies can keep enjoying life. Here is our guide to the Fish in cycle….

Emergency Fish in cycling 

Aquarium cycling is really important, it is what sustains the mini ecosystem that your fish call home.

Occasionally, aquarists find themselves inadvertently doing a fish in cycle. This means that the aquarium, or more accurately, the filtration cannot process the waste produced by your fish.


This isn’t an ideal place to be. Imagine being in a small room, with no way out, you eat there, sleep there and even do your business there.

Now imagine that your only source of ‘fresh air’ is a filter that sucks in your mess, pulling air through it and then blasting the same air, likely with smaller amounts of your waste, back into your little room.

Change air for water and that is what’s happening to your fish.


Nasty right?! 


There are many reasons why this can happen: over cleaning the filter, a dead fish not being seen, or the tank not being properly cycled to begin with are all common reasons. So, it’s handy to know how to do it as safely as possible.

There are loads of guides on how to perform a fish in cycle in the ‘safest possible way’, we’ve shared one previously from Kev at Kavman aquatics.

But not all of them are the best advice to follow, and I’ll explain why.


If you’re new to aquariums then our full guide to cycling is a good place to start, then come back here if you need to. You can find it HERE.


Ultimately, fish in cycling always carries risk, so rather than a ‘safe’ way, what I will tell you is the ‘safest way’ to do it. So here goes…


fish in cycle fish in cycling


Fish in cycling with Seachem Prime

Many sites have guides to fish in cycling that involve using Seachem Prime. If you read the Prime bottle then this seems like a decent plan, as it states that it can ‘bind ammonia and nitrite’ for up to 48 hours.

Seachem’s site says the same thing.

Now, I must say that I love many Seachem products and Prime is no exception…….BUT…..


There is absolutely no evidence that it can bind ammonia or nitrite. Even Seachem couldn’t tell me how it works.


Many have tested it and found that it doesn’t work, so far no one has proven that it does. Many aquarists believe this statement though and will recommend this method. I can’t for the above reason.

After searching through many, many, many documents on google scholar I cannot find any substance that would bind ammonia that is present in prime, or in an aquarium for that matter.


fish in cycling fish in cycle


Fish in cycling the safest way

So, if prime doesn’t ‘do what it says on the tin’, how can we safely perform a fish in cycle?

Well, the answer is simple and there is only really one solution.

The good news is, you will only need a few items to do it:

  1. A water testing kit – Like THIS ONE.
  2. A bucket or a hose (hose is much easier – SEE HERE.
  3. A bottle of dechlorinator, you can even use Prime! Our top recommendation is Quantum, available HERE.
  4. Patience

And that’s it!

The safest way to perform a fish in cycle is with regular testing and water changes.


What am I testing for?

Fish waste produces ammonia, which is converted to nitrite, and then nitrate. Ammonia is the most toxic, nitrite slightly less so and nitrate much less so than that. With your trusty testing kit, you’ll need to check your ammonia and nitrite levels regularly.

  • Ammonia needs to be kept under 0.25ppm.
  • Nitrite needs to stay under 1ppm.


This is where your water changes come in. For a full guide on how much to change, SEE HERE.

But for ease, a 50% water change should have your levels, 75% reduced them to a quarter of what they were, and so on.

You will need to keep these levels this low until your filter cycles, or re-cycles, depending on how this situation arose.

Fish in cycling, and keeping levels this low means lots of water changes, they’re usually required daily, depending on your stock levels.


fish in cycling


How do I know when it’s cycled?

Your aquarium is considered cycled when it can process the bio load that it contains.

That means that your tests will no longer detect ammonia or nitrite but will show rising levels of nitrate.

So, in this case, when your tests stop detecting both ammonia and nitrite, it’s done.


Where do I go from here?


Once this happens, your aquarium can be considered cycled and you can safely revert to a normal maintenance schedule.

That means that your water changes are now only dictated by your nitrate levels and in some circumstances KH levels. For a full guide, take another look HERE.

Give it some time before considering adding any more fish. This will allow your colonies of nitrifying bacteria to grow large enough to support a larger bio load.


Short-cuts to fish in cycling


Fish in cycling takes time, in some cases a long time, but there’s a shortcut and it’s the same shortcut for starting a fish-less cycle:

Below are two ways to drastically cut down the time it takes to cycle your filter:

  1. Used filter media: taking filter media directly from an established, healthy filter will immediately add live bacteria to the new filter when placed inside it. The more used media you add, the quicker it cycles. If you add enough (fill the filter with it) then it can be cycled instantly.
  2. If you don’t know anyone willing to give you some used filter media then see if anyone will part with the ‘horrid brown gunk’ from their filter instead. As disgusting as it may be, it is filled with bacteria that will significantly speed up the cycling process.


Throw either of these into your tank or filter and it will speed up the process drastically. How much depends, mostly, on how much you add.


These are the only real, reliable ways to speed up aquarium cycling. Bottled bacteria or Bacteria balls may help, but I haven’t fully tested them so I can’t comment on their reliability or effectiveness.


fish in cycling fish in cycle


Finally, good luck!

Performing fish in cycling takes quite a bit of effort and work.

If you’re lucky and keep the ammonia and nitrite levels as low as possible then your fish may well survive it.

But, unfortunately, there is no guarantee, so all I can do from here is wish you luck.


Had to do this? Let us know how it went in the comments.

fish in cycle fish in cycling

About the author

James @Seriously Fishy

I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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