How to Cycle A Fish Tank

Tropical fresh water aquarium front view with lush foliage plants and some fishes yellow Pterophyllum Scalare and Cardinalis neon
Gregory_DUBUS / Getty Images

Every new aquarium has to go through the tedious process of cycling. "Cycling" refers to establishing bacterial colonies that regulate your nitrogen cycle, the conversion of ammonia to nitrite to nitrate. There is no way to instantly establish the necessary stable nitrogen cycle in a new aquarium. However, if you take the time to properly set up your filtration, your fish will thrive in their new home. With all of the techniques described below, use a liquid-based test kit to make sure your levels are where you expect them to be.

In order to protect your fish from a sudden crash, always start with 10 to 20 percent of your final fish load. This will allow your filters to establish without putting your fish at risk. Slowly add fish over a few weeks to months, keeping a close eye on your water quality as you go.

Do you need to take steps to make sure your filter is established before you add fish? No. If you start with a completely brand new system and have no access to other systems and don't want to go with a fish-less cycle, you don't have to. Start with a low bioload and watch your parameters carefully. Expect to see spikes in ammonia, nitrite and nitrate as your filter becomes established. By keeping only 10 to 20 percent of your total fish load to start, your ammonia and nitrite spikes will not be deadly. Test your levels regularly and get ready to do some extra water changes if your levels creep up too high. In four to six weeks, your tank will be cycled.

Fish-less Cycling

The benefit of this method is that no fish are required to be in the tank; therefore, no fish are at risk of being lost. Timing is very important when doing a fishless cycle. If you are ordering your fish online and not at a store, be sure to contact them to know exactly when your fish will arrive.

The key to a fish-less cycle is that YOU are responsible for adding ammonia to the tank to replicate the waste that would be produced by the fish. You must continually add ammonia to your system so your cultured bacteria do not die from a lack of food. Once your filter is cycled, you will need to add fish slowly to keep the system going.

Fish-less cycling will not cycle the tank instantly. Just as the standard methods of establishing biological colonies in an aquarium take time, so does fish-less cycling. However, it is certainly easier on your fish.

Using Pre-Established Media

Also known as "seeding an aquarium," using pre-established media from another system is an age-old method for jump-starting the biological colonies in a new aquarium. Use a portion of the filter media from a previously established system or a cup of gravel from a stable, established tank in a bag and put it into the filter. Transfer between systems should take place quickly, without allowing time for the bacteria to die or dry out. Unless you have a lot of filter media to spare, this method will shorten your cycling process, but allow some time for your filter to reach full capacity.

However, not all bacteria colonies will work in different tanks. Do not use seed material from an aquarium that has dramatically different water parameters than the new tank. Test your water to make sure the pH and kH is in a similar range. Like many fish, bacteria cannot withstand dramatic shifts in water chemistry. All fish will bring some of their own bacteria into the mix. The colonies you start out with might not be what you end up with. Keep a close eye on your nitrogen parameters to make sure things are progressing smoothly.

Never take bacteria from a tank showing any signs of disease. Even though most pathogens only live on fish, there is a chance that something might be lurking in their filter colonies.

Dual Filters

Another method to speed up the nitrogen cycling process is to place the filter that is to be used in the new tank on an already established tank and let it run side by side with the existing filtration system. This method will require some advanced planning on your part. As with the method above, make sure your systems are healthy and have similar water chemistry parameters.

After 4-6 weeks, this matured filter can be moved to the new system. Make sure everything is ready to go before you transfer the new filter! Don't forget to add a few fish so your filter has some ammonia to eat.

Over-the-Counter Bacterial Products

Many fish stores will sell you a bottle of bacteria to "instantly" start your filter. Provided that what is in the bottle is still viable, the chance that those bacteria colonies are what your fish need is slim. Save yourself the money and take some time to start your filters correctly. Avoid the temptation of instant gratification and your fish will thank you.

Regardless of the transfer method used, there is no magic method to immediately establish fully functional biological colonies. You will need to plan accordingly and take your time. Test your water chemistry regularly until your system is fully cycled and stabilized.