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Cycling a New Aquarium

Updated on February 16, 2013
The nitrogen cycle / pathway you need to set up in a new aquarium.
The nitrogen cycle / pathway you need to set up in a new aquarium. | Source

The bane of any new aquarist's existence is New Tank Syndrome. People get excited about a new tank for themselves or their kids, then pay a fortune in a new tank and equipment of a birthday present. They come home with their new fish, fill up the tank and dump the fish and plants in. Within hours to days, one fish floats to the surface... then another, and eventually the tank is empty and sitting in the garage.

The worst way to get into any new hobby is to have massive failure right off the bat. The way to avoid that happening through New Tank Syndrome is to understand what the syndrome is and how to avoid it.

Any new tank needs to be “cycled.” This means establishing an ecosystem of beneficial bacteria which will process fish waste into relatively harmless nitrate from deadly ammonia. It takes understanding of the process and patience.

Freshwater Aquarium Setup Tips

When you cycle a new aquarium, you're allowing the beneficial bacteria to grow on the substrate (gravel), plants and in the filter. This will help remove toxins from the water even better than the charcoal filter.

This process needs to be done anytime you're setting up an aquarium from a dry state. This is true if the aquarium is straight out of the box or a used aquarium that you brought home or from storage.

Compared to streams, lakes and oceans where fish naturally live, the aquarium is a closed system. There are no streams, rivers or currents to wash pollutants away and bring in fresh nutrients. You add and subtract everything from the aquarium. Fish poop, decomposing food and expelled waste from gills turn into ammonia in the aquarium water. This has to be removed to keep your fish healthy.

What is the Aquarium Nitrate Cycle

The way the ammonia is processed away is called the Ammonia Cycle or Nitrogen Cycle. It's really a pathway, not a cycle. You have to remove the end product through your weekly water changes. Bacteria eat the ammonia and turn it into nitrites. Other bacteria eat the nitrites and turn them into nitrates. The handy aquarist (you) essentially remove the nitrates into a bucket and replace 10 to 15 percent of the nitrate-laden water per week with fresh de-chlorinated water.

These beneficial bacteria grow on every surface of the tank. But much of it will be in the “biological filter” part of the mechanical filter. Almost all modern aquarium filters have a charcoal replaceable filter media and a permanent coarse filter or rotating wheel that will provide a place for much of those bacteria to live and thrive.

Aquarium ready to be filled and cycled
Aquarium ready to be filled and cycled | Source

How to Use a Fishless Cycle

Recently, the most popular method of cycling a new or reused fish tank is the “fishless cycle.” This method is considered more humane. You're not putting a sacrificial fish in a bacterialess tank to possibly die or at least suffer significant ammonia stress.

This method is easier and more effective if you have water or sand from an established tank with a similar fish species population as you are going to put in the new tank. This lets you import some of the bacteria. It's absolutely vital the donating tank has a healthy fish population. You could be importing disease and negative bacteria with the cycle bacteria.

Place the donated rocks into a mesh bag and hang it in the tank or place it in the mechanical filter to inoculate the tank. Every day or so, add a little bit of flake fish food, which will decay into ammonia to feed the bacteria. You can also use pure bottled ammonia. You must be extremely careful to slowly and consistently add artificial ammonia to avoid starving your bacteria by accident. It's easy to go too low on ammonia and have to start over again.

Live plants can also be planted in aquariums because they help process waste. Some carefully planted aquariums can actually avoid having any water changes at all!

Use the heater to keep the temperature at the level you will need for your intended fish species. This will make sure the right types and amounts of bacteria will grow that can keep going once you put in your fish. This is another reason not to use a gold fish to cycle an aquarium intended for tropical fish. It will also help if you get every air stone and air pump you can get your hands on and put them in the cycling tank. Agitating water cycles faster.

If you have test bottles for ammonia and nitrates, test and wait until you see ammonia / nitrites declining and nitrates building (see the amazon links to the right for test kits, note: affiliate links). Once this is the case, slowly add fish, about 25% of your target population at a time. Keep testing to make sure you don't overload your new bacteria buddies.

Fish Tank Setup Cycle with Hardy Fish

Some people still use hardy fish to cycle tanks, which has its benefits as well. A live fish will issue a steady stream of ammonia, which will be better for growing a robust bacteria bed faster. You also don't have the risk of accidentally starving your bacteria by missing a bottled ammonia dosage.

It is very important to not use goldfish to cycle a tank unless you are only going to be keeping goldfish in the finished tank. Not only is the temperature target different, but goldfish are themselves exceedingly dirty fish. They will quickly overwhelm the baby bacteria and harm themselves. They also have different kinds of waste than tropicals will expel, so the bacteria may not even be the right kind.

Zebra danios and mollys are hardy tropicals. You can put one fish per 10 gallons of aquarium size. That amount of bio mass will still create enough waste to create enough ammonia to start the cycle (and stress the fish), but likely not enough to kill them. More fish than that in an uncycled tank will create more waste, and thus more ammonia. It will create extremely painful ammonia stress on all the fish, which is extremely inhumane for your new finned friends. And remember that your total fish population should be no more than one inch of adult size per gallon of water. Your fish store tank labels will often give adult size in inches, and you divide the amount of water in the aquarium (considering that your gravel substrate and decorations will likely reduce water capacity by 10%) to determine how many fish you can host.

While you are cycling the tank, you should change approximately 10 percent of the water every five days. Once the tank is cycled, you should continue to do that every week. You can test the water yourself with ammonia test kits. Some fish stores and pets stores will test a sample of water for you at little or no cost.

Also remember to treat your water at the beginning and the water you add during changes with a dechlorination treatment. Chlorine kills bacteria, which is why your water district adds it. However, that's exactly why you don't want it in your aquarium. All that hard work cycling will be for nothing if chlorinated water wipes out your hard-earned bacteria.

No additional chemicals to cycle

Some stores and online sources want you to add cycling chemicals or 'bacteria in a bottle.' You want a stable naturally maintaining bacteria ecosystem. Adding bottled bacteria or unnaturally changing the ammonia levels with anti-ammonia additives midstream will collapse your cycle. This will be bad for your fish – and your wallet – in the long term.


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