Hardy Fish Best For Cycling A New Tank (Nitrogen Cycle)

Hardy Fish Best For Cycling A New Tank (Nitrogen Cycle)

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Each fish tank needs to go through a nitrogen cycle meant to establish a colony of beneficial bacteria both in the filter and substrate. These colonies help remove waste such as ammonia in the water, protecting fish from toxicity and early death.

If you are a seasoned fish keeper, you’ll know there are several ways to cycle a new fish tank, but by far, the most popular and a classic traditional method involves introducing hardy fish into the aquarium gradually.

The idea is for the fish to create waste in the tank, which then triggers the nitrogen cycle and encourage the development of bacteria, domiciled in the filter and substrate, to break down the waste into less harmful nitrates; see more here.

That said, not all aquarium fish are tough enough to endure the condition in a new (cycling) tank. In fact, only a handful of species are recommended. They include hardy minnows such as danios, white clouds, barbs, platies, mollies, and even goldfish.

Please note that it’s possible to cycle your fish tank without adding fish (recommended), which is by far more humane. So if you choose fish-in cycling, it should entirely be based on preference and not a scarcity of alternatives..

Read on for more insight.

Hardy Fish for Nitrogen Cycle

The Nitrogen cycle is pretty simple to implement. You will only need to monitor the ammonia and nitrites levels using commercially available test kits and maybe contend with a couple of unfamiliar hacks such as the right fish to use.

Overall, small, affordable species are more recommended, though not all can be used. For instance, you don’t want to add neon tetras in a cycling tank, albeit having a small body and being readily available.

From experience, any of the fishes outlined below will survive a nitrogen cycle if implemented properly:

  • White cloud mountain minnows
  • Zebra Danios
  • Mollies
  • Platies
  • Barbs

#1&mdashWhite Cloud Mountain Minnows

White cloud mountain minnow is a hardy species of freshwater fish that prefer things on the cooler side. They have a small body size, are readily available, and not too expensive, making them an ideal choice for fish-in cycling.

Depending on the size of your fish tank, a school of at least 6 individuals will produce enough ammonia to get the nitrogen cycle underway.

However, once the tank is fully cycled, it will only be able to deal with minnows at the beginning, so don’t stock it too heavily. Start adding other fishes to what you have gradually to allow the filter enough time to catch up.

#2&mdashZebra Danios

There is a belief within fish keepers circles that zebra danios are so hardy they can survive in a not-cycled fish tank.

Well, while I might not go as far as to try this out, I know the fishes are at least hardy enough to get through the new tank cycling process.

However, do not add extra (store-bought) ammonia when doing fish-in cycling with danios (or any other species). Just let the waste produced by the fish get the job done.

Adding quick start nitrifying bacteria or sourcing some from an old filter sponge is ok though, and actually recommended to hasten the process.

Same as white cloud minnows, zebra danios have small bodies and a fairly light-bioload. They are also affordable and readily available, so you should be able to get some for your cycling tank.


Mollies are tough, easy to keep, treat, and catch if you want them out of your tank. Plus, they can handle a higher level of salinity than most freshwater species.

As such, they are a popular choice for fish-in cycling, including in saltwater setups (black mollies), though more popular among traditional fish keepers😎😃 than newbies.

They are one also one of the few fishes you want to add to a recently cycled tank since they’ll have a better chance of survival in case the parameters are not fully stabilized.

Since mollies are not too tiny, two or three individuals should be enough to start the nitrogen cycle in a medium-sized aquarium. Any more, and you just might be asking for trouble.


Platies are a lot like mollies. Apart from both being livebearers, they are hardy, have the ability to adapt to varying water conditions, and don’t require much care.

So, when looking for a fish that can survive in a new or cycling tank, you should probably consider if you cant access mollies or danios.

A lot of people actually use platies, though there is a chance of a shorter life expectancy because of exposure to high ammonia and nitrite levels.

As such, test your water every other day and do water changes as needed to reduce lethal toxin doses.


Barbs are another hardy species of tropical fish that you can use to cycle your fish tank.

Since they prefer being in a school, add more than one in your tank. Just remember to do it gradually and ensure there are no spikes in your water parameters.

Plants are also recommended to help absorb excess ammonia that would otherwise kill your fish.

Final Thoughts

Cycling your tank with fish in it is a traditional trick that works most of the time, but the method is considered cruel and inhumane to your fish.

Over the few weeks that your aquarium will be going through the nitrogen cycle, your aquatic pets will have to contend with toxic ammonia and nitrite spikes with a less than efficient filter to remove them.

As such, fish exposed to these conditions will have a shorter lifespan assuming they even manage to survive the full cycle.

So instead, consider adding chopped up fish food such as shrimp from the pet store to trigger the ammonia cycle in your aquarium.

Alternatively, you can use bottled bacteria like Seachem stability and nitrifying bacteria such asAPI quick start to cycle your tank.

However, keep in mind that not all products work, so read every review and get advice from more experienced fishkeepers in your area or reputable fish store.

Thats all.

Happy fish 🐠🦐 keeping.

Eddie Waithaka

Resident Content Creator and Marketer at AquariaWise who talks about aquariums and fish and aquascapes a lot.

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