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How to Safely Cycle Your Fish Tank with Starter Fish.

Earlier, I wrote an article about how to fishless cycle your aquarium. This method is safer and less harmful to the aquatic inhabitants of your tank than cycling with fish. However, it is possible to safely and ethically cycle your new fish tank with living fish, without harming them. The key is to start off and progress as slowly and purposefully as possible.

Although it is called a cycle, which means it will continue over and over for the life of your tank, in this case, “completing” the cycle, or "cycling your tank" refers to getting a reading of 0 ppm ammonia, 0 ppm nitrite, and < 5 ppm nitrate, with an established beneficial nitrifying bacteria colony to maintain those parameters.

Seeding your Tank

Before you add any fish or invertebrates to your tank, you must seed the aquarium with beneficial or nitrifying bacteria first. As fish produce waste, it breaks down into toxic ammonia and ammonium (NH3+, NH4-). The nitrifying bacteria you seeded will break down the ammonia and ammonium by pulling the nitrogen from those compounds and converting it to nitrite (N02-). Then a second type of nitrogen fixing bacteria will convert nitrite to less toxic nitrate (N03-). Without these bacteria present, any ammonia or ammonium released by the fish can quickly build up and become toxic.

Bacteria Sources

There are several ways you can add beneficial bacteria to your new tank. Regardless of the method you choose, be sure to have your biological filtration setup. The biological filtration is the habitat the nitrifying bacteria will occupy and grow on. Without it, the bacteria you added won’t be able to grow to appropriate population levels.

From an Established Tank

One source of beneficial nitrifying bacteria is some biological filtration or filter media from another established tank. Whether it’s one of your own tanks or a friend’s, you can add filter media from an established tank to your own. It is imperative that the tank donating the bacteria has no pathogens, snails, or algae issues. It is easy for algae cells, pathogenic bacteria, viruses, fungus, parasites, and pests to hitchhike on physical media. If you’re not sure what the health of the donating tank is, play it safe and choose a different source of bacteria.

Shelf Bacteria

This option refers to the bottled beneficial bacteria available online and at most local fish stores. Several companies produce a bottled solution that may contain live bacteria. I specify may in this case as the shelf life of these bottles are very short. Additionally, the percentage of live bacteria in the bottle decreases over time. If you add a nearly expired bottle, with some live bacteria, to your new tank, they will feed off the ammonia produced by the decaying bacteria also in the bottle. This may produce an ammonia spike, but is not a sustainable source of ammonia/ammonium.

Adding Fish

Timing is crucial when cycling your tank with fish. Any organic material that decays will release ammonia and ammonium. If you don’t add fish fast enough, there won’t be a continuous source of ammonia and ammonium, causing the bacteria to die off. If you add fish too fast, there won’t be enough bacteria to keep up with ammonia and ammonium production, causing toxic concentrations.

Test First

After you add your beneficial bacteria, wait 12-24 hours and test for ammonia and ammonium. Write down what the concentration is, then retest at 24-36 hours. If the ammonia tests below .25 ppm and is not rising, you can add fish. If either test shows above .25 ppm, or the concentration rises from the first to the second test, do not add fish yet. Continue to test ammonia every 12-24 hours until ammonia is at or below .25 ppm and is not trending upward.

Starter Fish

I want to reiterate at this point, you can continue this process without adding fish and wait until your tank is completely cycled, meaning 0 ppm ammonia/ammonium, 0 ppm nitrite, and 0 ppm nitrate. It is less expensive, less nerve wracking, more ethical, and much faster than cycling with fish. Read Fishless Cycling to see how.

If you want to continue cycling with fish, and your ammonia is below .25 ppm and is not trending upwards, it's time to choose a starter fish. The purpose of the starter fish is to continuously produce ammonia as a source of nitrogen for the beneficial bacteria. Generally, you'll want to stick with one fish per gallon. Always err on the side of caution. If your cycling a 100 gallon tank, you may still only want to add five instead of ten fish.

Choose a Peaceful Fish

Let’s say you moved into a new apartment. Unlike most rentals, this apartment comes with a non-negotiable roommate. This roommate has been living in this unit since it was built and no matter how hard you try, no matter what technique you use, he won’t leave. For one reason or another, this roommate is here to stay. Every day you get up to make your coffee before work and the roommate chases you out of the kitchen. You try to sneak into the bathroom to shower and brush your teeth and the roommate jumps out from behind the shower curtain and scares you off. He slaps away and steals your breakfast, he hides your shoes, he yells, bites, and nips at you every waking moment. There is nothing you can do, you can’t leave, he won’t leave, your trapped with this terror forever. Your health begins to wane as your stress levels shoot through the roof. You become anorexic, lethargic, and sickly. You don’t know if you’ll make it another day let alone a week or a month.

This scenario is why we add peaceful and non-territorial fish to our tanks first. If the first fish we add is non-territorial, as in, they do not seek out and defend a defined territory or space, then they will not demonstrate any territorial or aggressive behavior on future additions. On the other hand, if the first fish you add is prone to keeping a territory, they won’t choose a small corner and be content. Like the roommate who was their first, they will consider the entire tank their territory and any newcomers as infiltrators into their territory.

Choose a Hardy Fish

New tanks can undergo drastic fluctuations in water chemistry parameters. It is important to start your tank off with fishes that can resist succumbing to elevated stress when parameters fluctuate, especially when the parameters include toxic chemicals like ammonia, ammonium, nitrite, and nitrate.

Fish that evolved in fluctuating environments like small streams and estuaries, or fish that evolved in extreme environments may be more hardy than others. However, the true testament of hardiness is captive bred. Fish that come from stock born and raised in captivity over several generations are going to be significantly more hardy than their wild-caught counterparts. Therefore, if you choose a species that is evolutionary predisposed to hardiness and has been captive bred, you have a recipe for success.

Peaceful and Hardy

Here is a list of some possible choices for freshwater and saltwater peaceful and hardy starter fish.

Saltwater Starter Fish

Freshwater Starter Fish

Ocellaris clownfish (tank-raised)


Lawnmower blenny


Percula clownfish (tank-raised)


Blue green reef chromis


Firefish goby (red or purple)

Buenos Aires tetra

Cardinalfish (black & white or pajama)

Cherry barb

Remember, there are plenty of fish that are either peaceful or hardy, that didn’t make the list because they are not both peaceful and hardy. The last thing you want is an aggressive roommate whose nearly impossible to remove.

Test Again

After you add fish, continue to test ammonia and ammonium, nitrite, and nitrate levels every 12-36 hours until you notice 0 ppm ammonia/ammonium, 0 ppm nitrite, and <5 ppm nitrate. If at any time your ammonia levels rise above .5 ppm, add zeolite to your filter media and perform a 25% water change.

Zeolite is a mineral that adsorbs the nitrogen in ammonia, reducing its concentration in your tank. You can also add Prime water conditioner to reduce ammonia toxicity. Also add aquarium (Epsom) salt to freshwater tanks to help reduce fish stress by improving osmoregulation.

After you add your first fish, be sure to feed sparingly. Any food you add gets converted to ammonia through the fishes’ digestion or by decaying directly in the tank. To reduce the chance of excess ammonia and ammonium, remove any leftover food that was not consumed. Feeding fish every other day at first is normal, and they will easily survive off that regimen. The exception to this is certain over-active species with high metabolism, or grazers that need to feed constantly.

Once your tank is cycled, you can add more fish. For the most part, do not add more than a couple fish every 10-14 days, until your tank has been established for several months.

Consider Fishless Cycling

So it is possible to cycle your tank with starter fish. I would implore you to consider the fishless method, however. It’s faster, safer, and healthier for your fish if you cycle your tank before you add livestock.





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