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The Nitrogen Cycle | Home Fish Aquarium Guide

Home Fish Aquarium Guide

Fishkeeping Information and Resources for the Home Aquarium



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The Nitrogen Cycle

What is this nitrogen cycle thing and why should I care?

Fish produce wastes in the water they swim in. These wastes are toxic and will quickly poison and kill your new fish unless the tank establishes colonies of good bacteria that can process and change these wastes into different substances that ultimately are less harmful to your fish. Even if your fish survive in waters with these toxic substances they will have a shorter, more painful life because of the wastes present in the water. You do want your pets to be healthy don’t you?

That’s why you need to know a bit about the nitrogen cycle.

To start out with we have this image of bacteria as being bad. Throw that thought out the window because not all bacteria are bad. There are some VERY beneficial bacteria in the world. Much the way plants take carbon dioxide (what we breathe out) and convert it to oxygen (what we breathe in), there are bacteria that can take fish wastes (ammonia) and convert them to other substances (first nitrites, then nitrates). Now, fish don’t consume nitrates, but they can tolerate nitrates at MUCH higher levels than either ammonia or nitrites. Additionally plants LOVE nitrates.

So, this is why you need to have test kits. Because without the kits you have no way of knowing what level of toxic materials are in your tank. Yes, you could look at the fish and see if they look okay, but really what more are you going to do, take their pulse?

I really think it’s a must to take Ammonia, Nitrite and Nitrate readings at regular intervals (or at the very LEAST Ammonia and Nitrite) until you are certain that the tank has cycled. It is really the only way to know if your biological filter is working properly. If you have kids I think it’s a fantastic opportunity for a learning experience – I did a quick spreadsheet where we could fill in the date and the levels of each of the readings.

Anyway, I really prefer the liquid test kits. These are the ones where you put 5 ml of water (usually) into a test tube and add a certain number of drops of a solution and then cap, shake and read the results (by all means read the directions as some of ours require a 5 minute wait before reading the results.) I’ve used several kinds of test kits, but my current favorite is by API because the colors are really easier to distinguish compared to the others that I’ve used.

I’ve used the “dip strips” and I think just as a quick spot check they can be okay, but it can be very difficult to identify exact readings (and I’ve seen discrepancies between them and other liquid test kits.)

Cycling a tank is the process of getting those good bacteria established and really there are a number of ways to do this. Traditionally it has been with a hardy fish (such as a danio), although this really isn’t a very humane approach. What you will see though if you do take this approach is the ammonia levels climb, nitrites remaining low or at zero. Then nitrite levels will start to rise, ammonia will drop (eventually to zero). Nitrite levels will peak and you will start seeing nitrate levels going up. Finally the nitrite levels will fall off to zero and you will have zero ammonia, zero nitrite and increasing nitrate in your tank. (Assuming there is nothing to consume nitrates like plants.)

When you see your ammonia and nitrites drop to zero the tank is considered cycled. (This is also why I think it’s important to test at regular intervals THROUGHOUT the process. If you test your water straight from the tap and have zero ammonia and zero nitrite that does NOT mean that it’s cycled, only after the levels have peaked and it has been processed to nitrates.)

From what I understand, it takes about 12 hours for the bacteria colony to double in size for the types of bacteria that eat ammonia and nitrites.

The typical period of time for unassisted cycling of a tank is about 4 to 6 weeks. Now you know why I suggested that patience was the big requirement in starting a new aquarium. There are a number of ways to speed up the cycle and even some humane ways to keep fish in the tank before the cycle is complete.

I will try to fill in information on all of the various approaches to cycling or speeding up the cycling process in the subpages listed below. I’ll also give details of at least one cycling process on one of our tanks.

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January 10, 2009 - 3:58 PM