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The Nitrogen Cycle


    The most important thing that a new aquarium owner needs to know is the nitrogen cycle.  This is a part of the setting up of the aquarium that many, including myself, did not know until it was too late.

    This story is true, and will show why it is important to understand this process:

    My sister got her first tank for Christmas.  It was set up and running the next day.  We waited a day before we bought fish.  This is a ten-gallon tank.  We got 5 swordtails and 5 flame tetras.  The person at the local chain store (LWM) told us that the tank would hold 10 large fish or 20 small fish.  Less than a month later, the fish began to die.  I began replacing them as they died.  I thought it was some type of disease, so we treated for ich.  There was no ich.  By the end of January, the fish were all dead.

    What could have happened?  The nitrogen cycle.  The fish began to die exactly when the ammonia or nitrite levels would be getting deadly.  I didn't know about the cycle, so I didn't test for ammonia.

    The next time we tried, we let the tank set with no fish for a week.  Then we went to a real fish store and we got 5 glowlight and 5 cardinal tetras.  Looking back, that was bad because that number in an un-cycled tank is just as bad as the first fish we got.  These survived.  Why did they survive?  The nitrogen cycle.  How?  The tank had probably finished the cycle during the week it set empty.  How do I know?  I don't.  I still didn't know about the cycle.  We were lucky.


    The nitrogen cycle is a very complex thing to learn.  If you are not prepared to learn it, you should reconsider keeping fish.  Some people may claim that the cycle is not important.  I am sure these people have had many mysterious fish deaths over the years that could easily be explained if they understood the cycle and how it works in an aquarium.  You may want to print this page so you will have it to read later.  When you buy a new aquarium and put your first fish into the tank, it will start a process called the nitrogen cycle.  This is known as “cycling the aquarium”.  This process will take up to 6 weeks.
    The Nitrogen Cycle starts the moment you put any organic material into your tank.  This can be water plants, fish, fish food, or pure ammonia.  For most new aquarists (fish keepers), it starts with fish.  Fish produce ammonia as a waste product.  Uneaten food that falls to the bottom or is taken up by the filter will also break down to form ammonia.  The ammonia can build up and kill the fish.  This is why everyone should have an ammonia test kit when they start a tank or add fish.  The good news is that there are bacteria that "eat" ammonia and turn it into a chemical called nitrite.  Nitrite is also deadly, and also builds up in your tank.  You should also have a nitrite test kit.  Once again, there are bacteria that "eat" nitrite and turn it into nitrate.  Nitrate is another chemical that can build up and kill fish, but that rarely happens because it takes huge amounts of nitrates to kill a fish.  They are usually kept low by weekly water changes, but a nitrate test kit is a good idea to check levels between water changes. 
    Unlike ammonia and nitrites, which can kill fish at levels above .5, nitrates are safe until they rise above 30ppm (parts per million).  At one time, this suggestion was to keep nitrates under 100ppm.  Due to recent discoveries I have made, along with information others have given me, I now believe nitrates may be more important than recently believed.  That means nitrates should never excede 30ppm for more than a week.  My own nitrates rose to 50ppm for a month and I began losing fish.  Once nitrates were brought under control by water changes, the mysterious deaths stopped.
    It should also be mentioned that in marine aquaria, nitrates are as bad as ammonia and nitrite.  This is especially true in reef tanks.  I don't have much knowledge of marine aquaria, but I do know that there are live rock and live sand that contain organisms that reduce nitrates.  For live sand, you can even use half live sand and half coral sand and the coral sand will slowly come to life as the live sand spreads its organisms.
    If you have fish in your tank during the cycle, it will not hurt to do 25% water changes every 3 to 5 days.  Don't touch the gravel or the filter during the cycle.  Most of the bacteria you will need grow in the gravel and filter.  It is very important not to use a gravel vacuum until the bacteria have fully developed in your gravel.  You should also avoid changing your filter’s cartridges during this time.
    It is my opinion that filter cartridges should only be cleaned when they become clogged to the point that they no longer allow water through them and water begins going out the overflow.  Filters should only be cleaned in dechlorinated water.  Filter cartridges should be replaced whenever they no longer hold up when in the filter.  Sponge type filters should never need replacing because they can be cleaned easily by squeezing them in dechlorinated water until they are clean.  Ammonia and nitrate removing materials are not really important filter media and should be avoided.
    There are various chemicals that claim to instantly cycle an aquarium, or to allow you to do water changes only twice each year.  There is no substitute for proper aquarium maintenance, but some bottles that contain live bacteria do work.  These bottles will normally be found in refridgerators, not out on the shelf with chlorine removers.  Water changes should still be done every week and should remove 25-30% of the water.  The best way to make a tank that will be stable long-term is to start with one or two hardy fish.  Fishless cycling has become popular in the last few years, but it isn't really practical.  Many people believe it is the best method, but it isn't natural.  Most people don't have the patience to wait for the fishless cycle to complete.
    Fish stocking should always be done slowly.  Even if there has been a fishless cycle, and the tank has fully developed bacteria, the aquarium will still require time to grow more bacteria to remove extra levels of ammonia produced by the fish.  When you first start stocking the aquarium, you should never put more than 1 inch of fish per 5 gallons.  The length of the fish is how long the fish will grow to, not how long the fish is when you buy it.  Fish do not grow to the size of the aquarium.  A pleco can grow to be well over a foot in a ten-gallon tank, if it survives long enough.  Bala Sharks grow to around 12 inches, but should be kept in tanks of over 100 gallons because they need the room to swim.  Also, you should start the cycle with the smallest fish that you will be keeping in your tank.  Do not start with goldfish (which do not belong in most aquariums less than 30 gallons and do not belong with tropical fish) unless you are going to be keeping a goldfish-only tank.  
    Even with this information, you still need to do as much research as you can before you buy your first fish.  You need to look at what your fish store has, and then research the fish that you find interesting.  That little two-inch fish may grow to be nearly two feet.  Two fish of the same species, such as two bettas or two gouramis, may attack and kill each other.  You should always know as much about a fish as you can before you buy it to prevent yourself from being surprised.