Sunday, February 25th, 2018

How to Cycle Your Fish Tank


Tank cycling is a very important step in establishing your new aquarium, and improper cycling of your fish tank is a common cause of failure among beginning aquarists.  “Cycling your tank” means that you are giving your aquarium enough time to run through the Nitrogen Cycle and establish a biofilter.

What is a biofilter?  Basically, a biofilter is a natural system made of colonies of bacteria that turn the harmful fish wastes in your aquarium into less harmful substances which can be removed during your regular water changes.  When you first setup your aquarium, you need to give it time to develop these naturally-occurring bacteria to process the ammonia produced by your fish into the more harmless nitrate.  This process can take up to 8 weeks, during which time it is important to not have too many fish in your aquarium, and to do weekly water changes (15-20% of the water).  Your chance for success is very slim if you setup your aquarium and add fish in the same day, or even the same week.

So how exactly do you cycle your tank?  Your objective is to grow certain bacteria to serve as your biofilter and make your tank fish-ready.  This basically means that your tank needs to complete the Nitrogen Cycle (Ammonia => Nitrites => Nitrates) before you add all your fish. Luckily, this process is fairly easy to accomplish if you have the patience to wait it out and not over-populate your aquarium at the start.

Six Steps to Cycle your Fish Tank

  1. Choose your biological filter
    The beneficial bacteria you want to grow in your aquarium needs plenty of surface area on which to grow.  For external filters, this would be sponges for power filters or canister filters, and grooved plastic balls for a wet/dry filter.  For internal filters, the biofilter could exist in sponges or in the form of an undergravel filter.  Indeed, an undergravel filter is one of the best mediums for establishing a biological filter.  For saltwater aquariums, the biofilter can also include live rock (the more the better!).
  2. Select a few (2-3) small hardy fish.
    It is important that you consider the hardiness of the species of fish you use during the cycling process, as they will need to be tough enough to withstand the high levels of Ammonia and Nitrites that may be present in your aquarium during the cycling process.  Some good hardy freshwater fish to introduce during the cycling process include guppies, platies, White Cloud mountainfish and zebra danios.  Popular saltwater fish include the damselfish, although the clownfish may be a more desirable species of fish to keep after the cycling process is complete.
  3. Test your Water for Ammonia (Days 1-15)
    During the first ten days of the cycling process, Ammonia will steadily grow to lethal levels while bacteria capable of converting ammonia to nitrate are growing.  In freshwater, these bacteria double in number about every 8 hours (about every 20-24 hours in Saltwater).  In about 8 to 12 days, the bacterial colonies will have grown to a sufficient number to dramatically reduce the ammonia levels in your freshwater tank to near zero (This process can take 15-20 days in a Saltwater tank).  During this time, you should regularly test your water quality, and do 15-20% water changes when the ammonia levels are the highest to reduce the stress on your fish.
  4. Test your Water for Nitrites (Days 20-35)
    After the ammonia has been reduced to nitrites (still a lethal substance to fish), the next set of bacteria capable of converting nitrites to nitrates will begin to grow.  The nitrite levels will usually peak in days 30-35, after which they will dramatically drop to levels near zero as the nitrite is reduced to nitrates.  During this time, you should regularly test your water quality, and do 15-20% water changes when the ammonia levels are the highest to reduce the stress on your fish.
  5. Final Water Change
    Once your nitrite levels have dropped, your tank has been properly cycled and is ready for you to introduce fish. Before you add your fish, however, you should test for nitrates, and conduct a final 50% water change if the nitrate levels are above (or close to) 20ppm (parts per million).
  6. Test your Water for Nitrates (ongoing)
    Once your tank has cycled, you should only need to test for nitrates on an ongoing basis. If you are conducting regular water changes (10-15% of the water volume every week), you should rarely get levels over 20ppm.  Fish can tolerate levels up to 40-50ppm, although I would recommend doing a larger (25%) weekly water change whenever your nitrate levels exceed 20 ppm.

Cycling your Tank Without Fish
The process of cycling your tank with fish may seem cruel to some, and is actually unnecessary.  I mention it here because I think it is easier for people to hold off on adding a bunch of fish to their tank if they (or their kids!) AT LEAST have one or two fish to look at.  And if you conduct daily 15-29% water changes when the ammonia or nitrite levels are the highest, your starter fish should not suffer from too much stress.

However, the cycling process can easily be accomplished without fish through a method called Fishless Cycling by adding a small amount of fish food (or ammonia) on a daily basis.  The food will decompose, releasing ammonia and kicking off the Nitrogen Cycle.

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