While it is not recommended to skip the cycling process I have done it a couple of times with some success. A cycle process that relies on traditional methods is going to be safer and give you a better experience with your aquarium in the long term. See here how to cycle a tank the traditional way.
What is cycling a tank?
Cycling a fish tank refers to the process of establishing a beneficial bacterial colony in a new aquarium. This colony will help to break down fish waste and uneaten food into less harmful compounds, such as nitrite and nitrate. The process of establishing this colony is called the Nitrogen cycle.
Why should you skip cycling a tank?
There are some legitimate reasons to skip cycling a tank.
For example if you have a sick fish and you need to set a quarantine tank you are not going to have enough time to cycle the tank or the aquarium. Another reason would be if you have a bunch of fry born and you need to put them in safe place to raise them.
Both of these reasons are why I started to skip the cycling process on some of my tanks. While I rarely lost any fish you need to carefully monitor the water quality and be prepared to make some water changes to keep you water quality within the correct parameters.
Different methods to skip cycling a tank
If you talk to experienced fish keepers you quickly find out that not all of them cycle a tank when setting it up for the first time. Especially if the tank is small and they are just using it for breeding or raising some fry.
Pre-Established Filter cycling Method
I like to use a filter from an established tank to start my small tanks. I have found that using a sponge filter or an air box filter filled with media from an established tank is one of the best ways to get some beneficial bacteria into the water column quickly especially with a 10 to 20 gallon tank. Using this method you could add fish in as little as one day as long as you do not add too many fish at once.
If your fish are smaller like guppies, and mollies you can safely add the fish and let them be as long as you have a good air stone that circulates the water within the aquarium.
Fish cycling method
While this method will not be as good as using a starter bacteria it can work. You do have to be prepared for your tank to crash at some point because the bad ammonia and nitrates will overtake the beneficial bacteria. When this happens you will need to do a big water change multiple times to stay on top of the water quality within your tank.
Start by setting up your aquarium as you normally would with the filter, substrate and decorations.
Once you have your aquarium set up go ahead and add your fish. These fish need to be fairly hardy like mollies and some barbs. You do not want to add too many fish at this point, I would recommend one fish per a gallon of water to get started. Any more than this and you risk crashing your tank.
Make sure the fish you add are healthy so you do not introduce any diseases to your tank.
Signs of a cycled aquarium
You can tell when your aquarium is fully cycled when your ammonia, and nitrite levels reach 0ppm and your nitrates are under 20ppm.
This means that you will need to test your water with a test kit multiple times a week until the levels have stabilized.