Cycle with Fish (fish-in cycle)

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, all new fish tanks must be cycled. There is more than one way to cycle a fish tank. While fishless cycling is highly recommended, cycle with fish can be successful too if it is done correctly. To start the aquarium nitrogen cycle, there must be a source of ammonia. The fish themselves are a source of ammonia since they produce it on a constant basis, therefore you can cycle the fish tank with fish. It is also known as the fish-in cycle.

To cycle with fish, you first need to set up an aquarium with all the necessary equipments and supplies.
1. A fish tank of 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 start a fish-in cycle
A. You have to set up an aquarium first.
Everything must be running before you add the fish.

B. The fewer fish the better.
You should only add as few fish 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 the cycle can be completed.

C. Choose your fish species wisely for the purpose of cycling the tank with them. The fish for cycling a tank must not all die before the cycle can finish. Traditionally people like to choose 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 the tank is cycled 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 and at the same time compatible for the future fish you plan to get, since you want them to survive long enough to complete the fish-in cycle.

D. It is highly recommended to use SeaChem Prime as your water conditioner. Since you will have fish in an uncycled tank, ammonia will definitely 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 the primary function of neutralize chlorine and chloramine in the tap water, SeaChem Prime has a secondary function of detoxify ammonia and turn it into harmless ammonium for up to 48 hours. This function alone can make certain the fish will not be exposed to ammonia poisoning within 48 hours after each water change with Prime. As long as you do partial water change with Prime every other day, the ammonia damage to your fish should be minimal.

E. 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, but rather it might kill the fish faster. Once the fish is dead, you can’t continue the fish-in cycle. Feeding lightly every other day is recommended during fish-in cycling.

Steps of Fish-in Cycling

Step I – To cycle with fish, first make sure the filter is running, the heater is giving the right water temperature, and the air pump is making bubbles. Add as few fish as possible into your prepared aquarium.

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

Step III – 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. This should ensure your fish will not 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 3 weeks before the next step.

Step IV – Once you pick up nitrite, you are at least a 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 daily basis. It might take weeks more to get a nitrate reading.

Step V – 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.

Step VI – The day you get a 0ppm reading on both ammonia and nitrite, you should stop doing partial water change every other day. Furthermore, you only need to do a partial water change once a week to lower the nitrate concentration. This 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.

Step VII
– Wait for at least 3 days in a row after you start to get 0ppm readings on both ammonia and nitrite. You may start to feed your fish every day instead of every other day. This will ensure normal production of ammonia by the fish, and as a result the beneficial bacteria will also increase in quantity.

Step VIII – Wait at least another 3 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. Too many fish added 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 fish-in cycle. If you have more than one fish survived the fish-in cycle, it is still recommended 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 is 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 this might be seen as an advantage for some people.

The disadvantage of cycling with fish.
Your fish will be exposed to ammonia poisoning during the fish-in cycle. The fish you use for cycling the aquarium might not survive.

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

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

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

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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