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The Nitrogen Cycle

Ammonia & nitrites are basically the waste chemicals that constantly accumulate from your fish population, but they are also the basis of the nitrogen supply in your system. Again, without resorting to a highly technical chemical lesson, there are a few things you need to understand.

Fish create ammonia as waste, as will any fish food that is left in the tank to rot. Ammonia is converted to nitrites by natural bacteria in the water. The nitrites are then used by another form of bacteria to become nitrates. It's this nitrate form of nitrogen that your plants are looking for, and what you need to have available in the water of your system. Unfortunately, chemically treated water that comes from most city taps won't have too much natural bacteria it it which can spoil your nitrogen cycle.

We've already mentioned that you should let the chlorine air out of your water before adding to the system, but you might need to add some of this bacteria while you're at it. For an established system it won't be necessary. This is just for new systems, or if you've found your nitrogen cycle has gotten out of whack. You can add some water from a local pond or a clean aquarium for this. If transporting actual water won't work, grab some gravel or a few rocks instead. Both will help.

So while we may talk about nitrogen as a single thing, there is more to the story because of the multiple steps. If you are low on nitrogen, it may be that you have too few fish (and that is the most likely reason) but there are two steps in the chain between ammonia and nitrate that you can't ignore. In other words, your nitrogen levels aren't dependant solely on the fish. A reduction in natural bacteria can mean your ammonia just builds up and is useless to your plants, not to mention that it will kill your fish.

This all means that testing for "nitrogen" is only one part of the equation. If your nitrogen (or more specifically, nitrates) are low, then get a few more specific tests and see what your nitrite and ammonia levels are at so you can find out which part of the cycle is your problem.