Question
I have read that prior to putting fish in an aquarium, one must "cycle" the tank so that bacteria that oxidize ammonia will produce nitrite, and nitrite-oxidizing bacteria will make nitrate. I understand the chemistry behind it, but I'm not really sure what is the best practical method to do this. I'd love to start an aquarium, but this task seems daunting.
Answer
First, a little background on the aquarium nitrogen cycle.
Fish produce ammonia, which is extremely toxic to the fish. Unlike in a natural body of water, in an aquarium there is nowhere for this toxin to go, so it builds up fast. Luckily, there is bacteria in your tank that converts ammonia into nitrite. However, nitrite is also extremely toxic to fish. So it is good that there is another type of bacteria in your tank that converts nitrite into nitrate. Nitrate is also toxic to fish, but not nearly as much so as ammonia or nitrite, which means it is easier to manage with water changes.
When you first set up your aquarium, the population of these bacteria is low and needs to be built up. So the goal of "cycling" your aquarium is to get the population of bacteria up to where it needs to be to keep your fish healthy.
One way this is done is to simply put cheap fish in your tank that will bring bacteria with them from the pet store. The problem with this approach is that the first several fish you put in your tank will likely die, as the population of your bacteria has not grown yet. Your cycling is complete when the fish stop dying.
Instead, I have had great results with fishless cycling. The idea is that you set up your aquarium with the heater and the filter, but before you put any fish in it, you add drops of pure ammonia to feed the bacteria. Then you test the water every day for ammonia and nitrite to check the progress of the bacteria population. When you get to a point where you can add ammonia, but the ammonia and nitrite are quickly processed by the bacteria, then your tank is ready for fish.
There is a great article on fishless cycling with details on a good procedure here: Cycling Your First Fresh Water Tank
The whole process can take a few weeks, and it can be sped up by introducing something from an established aquarium, such as a decoration or a plant, that will bring some bacteria with it. However, if you do bring something over from another tank, make sure it is a healthy tank, as you can also transport diseases to your new tank.
Answered By - Ben Miller - Remember Monica