Cycle with Fish (fish-in cycle)

Cycle with fish in the aquarium is one of the options of fish tank cycling.  It is also known as the fish-in cycle.

Why we must cycle fish tanks

Aquarium nitrogen cycle is essential for the survival of pet fish in a home aquarium. No fish can live for long in a new aquarium before it is cycled because the toxic ammonia will build up from decay organic matters in the aquarium as well as from fish gills directly.

To ensure an ammonia free aquarium, we must cycle all new fish tanks. There is more than one way to cycle a fish tank. While we highly recommend a fishless cycling.

We must have a constant source of ammonia to start the aquarium nitrogen cycle.  The fish themselves are a source of ammonia since they produce it on a continuous basis. Therefore you can cycle the fish tank with fish.  Cycle with fish can be successful only if we do it correctly.

To cycle with fish, you first need to set up an aquarium with all the necessary equipment and supplies.

1. A fish tank of the proper size for your fish.
2. An adequate aquarium filter system.
3. An aquarium heater.
4. An aquarium air pump and accessories.
5. An aquarium water conditioner.
6. A few 5-gallon water buckets.
7. A gravel vacuum.
8. An aquarium water test kit.

Things to do and to know before starting a fish-in cycle

You have to set up an aquarium first

Everything must be running before you add the fish.

The less fish, the better

You should add as few fishes as possible. More fish will not cycle the tank faster. Instead, more fish means quicker ammonia building up, and the fish will be more likely to die before it can complete the cycle.

Choose your fish species wisely

To cycle with fish, you must choose the right fish. The fish for cycling a tank must not all die before the cycle is complete. Traditionally people like to select a few “expendable” hardy fish to cycle an aquarium.

Zebra Danios are known for being able to survive in terrible water conditions for a while, and therefore they are one of the most common fish-in cycle candidates. However, Zebra Danios are known nippers. They are not compatible with longfin and slow-moving fish. If you have the plan to get such fish after cycling the tank without a plan to get rid of the Danios, you should use another species instead. Platies and guppies are also often used for fish-in cycling. Whatever fish species you plan to use for cycling a tank, they must be hardy.  At the same time compatible with the future fish you expect to get since you want them to survive long enough to complete the fish-in cycle.

Use an aquarium water conditioner that can detoxify ammonia

It is highly recommended to use SeaChem Prime as your water conditioner. Since you will have fish in an uncycled tank, ammonia will build up. Any trace of ammonia is harmful to the fish. Therefore the fish need your help to live through the fish-in cycle. Aside from the primary function of neutralizing chlorine and chloramine in the tap water, SeaChem Prime has a secondary function of detoxifying ammonia and turn it into harmless ammonium for up to 48 hours. This feature alone can make sure the fish will not be exposed to ammonia poisoning within 48 hours after each water change with Prime. As long as you do a partial water change with Prime every other day, the ammonia damage to your fish should be minimal.

Feed as little as possible during the fish-in cycle.

More fish food means more ammonia in the system. More ammonia will not help the fish-in cycle.  Instead, it might kill the fish faster. Once the fish is dead, you can’t continue the fish-in cycle.  We recommend feeding lightly every other day during fish-in cycling.

Steps of Fish-in Cycling

Make sure everything is working properly

To cycle with fish, first, you must make sure the filter is running.  The heater is providing the right water temperature, and the air pump is producing bubbles.

Add Fish

Add as few fishes as possible into your prepared aquarium.  Too many fish can produce too much ammonia which will kill the fish rather quickly.  The whole idea of the cycling with fish is to have the fish stay alive until the cycle is complete.

Test the water every day for ammonia and nitrite

Test the water every day for ammonia and nitrite. Since the tank is uncycled, you will be able to pick up a small trace of ammonia reasonably quick (usually in a day or two). If you have used SeaChem Prime as the water conditioner, the reading you get should be ammonium instead of ammonia.

Partial water changes every day

As soon as you discovered ammonia or ammonium in the water, you need to do a 30~50% partial water change every other day with Prime as the water conditioner.  By doing so, it should ensure your fish won’t die to ammonia poisoning before the aquarium cycle can finish. It will also keep the ammonia/ammonium level in check since more ammonia/ammonium will not make the aquarium cycle faster. It can take more than three weeks before the next step.

Test for nitrate

Once you pick up nitrite, you are at least one-third way into the cycling. It is a good sign, but do not cut back on partial water changes. From this point on, you should also test for nitrate in addition to ammonia and nitrite tests on a daily basis. It might take weeks more to get a nitrate reading.

Reduce the partial water change

Once you get a reading on nitrate, the final stage has arrived. At this point, you still need to do it every other day with Prime, but you can cut back on the percentage of water changed. For example, if you were doing a 50% partial water change every other day, now you may reduce it to 20% instead.  It is due to the fact you already have plenty of bacteria, and they have an increasing appetite for ammonia and nitrite.  Too much partial water change will limit their food source.  Therefore, it will slow their growth.

Reduce partial water change down to once per week

The day you get a 0ppm reading on both ammonia and nitrite, you should stop doing the partial water change every other day. Furthermore, you only need to do a partial water change once a week to lower the nitrate concentration.  It is to ensure the developing beneficial bacteria to have enough ammonia and nitrite to convert. The goal at this stage is to get 0ppm on both ammonia and nitrite.

Continue test for ammonia and nitrite, and start feeding fish every day

Wait for at least three days in a row after you begin to get 0ppm readings on both ammonia and nitrite. You may begin to feed your fish every day instead of every other day.  It will ensure regular production of ammonia by the fish, and as a result, the beneficial bacteria will also increase in quantity.

Cycle with fish is complete

Wait at least another three days before you can finally call it “the tank is cycled” if you continue to get 0ppm ammonia and 0ppm nitrite readings. Congratulations!

You may add more fish now but do it slowly

Introduce too many fish at once can still produce ammonia spike in an aquarium you just cycled with fish. It is a good idea to add no more than one fish at a time if you had only one fish in the tank by the end of the fish-in cycle.

If you have more than one fish survived the fish-in cycle, it is still a good idea to add only 1/3 of the existing number of fish at a time. In both cases, please wait for at least a week or two before you add more fish. You must also continue to do testing on the aquarium water for ammonia, nitrite, and nitrate. If there is any sign of ammonia or nitrite, you must immediately do a partial water change with Prime, and repeat it every 48 hours until there are no more ammonia and nitrite.

The advantage of cycle with fish

You get to have the pet fish immediately after you set up your aquarium. Many people can’t wait to get their hands on the fish the moment they set up their aquariums.  Therefore, we can see it as an advantage for some people.

The disadvantage of cycling with fish

You will expose your fish to ammonia poisoning during the fish-in cycle. The fish you use for cycling the aquarium might not survive.

There is a lot more workload because of the frequent partial water change required to sustain the fish through the tank cycle.

Cycle with fish is usually slower than fishless cycling. You will, in fact, wait longer before you can fully stock your fish tank.

There is no exact set time for the fish-in cycle just like fishless cycle due to too many variables in different tanks. You can certainly speed the process up and cut down the time required by applying some fish tank cycling tips. Although not guaranteed, some of the suggestions might cut down the aquarium cycle time to as short as a week or just a few days.

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3 thoughts on “Cycle with Fish (fish-in cycle)

  1. Talisa Appleby

    Thank you very much!

    I followed your instructions with a few additions of my own (like a biomedia filter inside my tank).

    I lost no fish during this process (a Cichlid and a Pleco).

    After 3 weeks, my tank is “officially” cycled!


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