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
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Every new aquarium has to go through the tedious process of cycling. "Cycling" refers to establishing beneficial bacterial colonies that regulate the nitrogen cycle, the conversion of ammonia to nitrite, then 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 the water levels are where you expect them to be.

In order to protect your fish from a sudden water quality crash, always start by stocking your aquarium with only 10 to 20 percent of your final fish load. This will allow the beneficial bacteria to grow in the filter media without putting your fish at risk. Slowly add more 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. However, if you have filter media from an existing aquarium you can add some to your new aquarium filter to 'jump-start' your biofilter. If you start with a completely new filtration system and have no access to filter media from other systems, start with a low bioload and watch your water quality parameters carefully. Expect to see spikes in ammonia, nitrite and nitrate as your filter becomes established. By adding only 10 to 20 percent of your total fish load to start, your ammonia and nitrite spikes will not be deadly. Test your water quality 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 aquarium; 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 aquarium 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 nutrients. This can be done by adding ammonium chloride powder daily in small amounts and measuring the ammonia and nitrite levels. The ammonia will initially get higher, but as the beneficial bacteria start growing, the ammonia level will go down and the nitrite level will increase. When the nitrite level starts going down and the nitrate increases, then the aquarium has been 'cycled'. 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 media from another pre-established filtration 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 mesh 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 will still take some time for your filter to reach full biological 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 (acid-base balance) and kH (alkalinity) are 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 filter media or substrate from an aquarium with fish showing any signs of disease. Even though most pathogens only live on fish, some can survive in the water or gravel and there is a chance that something might be lurking in the filter media.

Dual Filters

Another method to speed up the nitrogen cycling process is to place the filter that is to be used in the new aquarium on an already established aquarium 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 fish are healthy and the aquariums have similar water chemistry parameters.

After 4-6 weeks, this matured filter can be moved to the new aquarium. 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. It still takes time for the beneficial bacteria to grow even if you add a commercial bacteria supplement to the new aquarium. 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 cycling 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 filtration system is fully cycled and the water quality is stabilized.