The Aquarium Nitrogen Cycle

How Mother Nature Cleans House
By Alan Ruben
First published in The Daphnian, Boston Aquarium Society, Winter 2002

Lets start off really basic. How far you get along into this is up to you.

Fish make Waste – Bacteria get rid of Waste.

Complex enough for you? OK, we can get a little more specific.

Fish produce ammonia (NH3) as the majority of their waste. Ammonia is toxic to fish so we wouldn’t want that to build up in our aquarium. Luckily, there are types of bacteria that use Ammonia as part of their metabolism.

They use up the available ammonia and convert it to a compound known as nitrite (NO2). So, all is safe and sound, right?

No. Unfortunately, Nitrite is also very toxic to fish.


Now it happens to be that another type of bacteria come along and use this Nitrite in their metabolism. They scrounge up all the Nitrite they can find and convert it to a compound known as nitrate (NO3). So all is safe and sound?

YES! Well for the time being…

Nitrate is MUCH less toxic than either Ammonia or Nitrite but should not be allowed to build up. SO…the way to keep the Nitrate level down is…Water Changing.

I knew you didn’t want to hear that but that’s the way it is. For the majority of aquariums, water changing is just part of the scenery. No way around it. Anyone who tells you otherwise is trying to sell you something (in my not so humble opinion).[su_posts template=”templates/teaser-loop.php” posts_per_page=”1″ tax_term=”1″ order=”desc”]

Got the concept? Want more specifics? Are you sure? Ok, you asked for it.

When first setting up an aquarium, the tank and all of the contents inside of it are relatively clean and free of bacteria, especially the kinds that we want. An aquarium takes time to establish a colony of “bugs” to get at the nasty wastes. This is what we call a Bacteria Bed, or also a Bio-Filter

  1. A new aquarium gets set up. (assume water etc. is ok)
  2. Fish get introduced.
  3. Fish get fed ( I would hope).
  4. Fish swim happily in enjoyment of their new home.
  5. Fish produce waste.
  6. Ammonia from the waste starts to build up (TOXIC).
  7. Bacteria start growing that use Ammonia.
  8. Ammonia level starts to drop as Nitrite level builds (TOXIC).
  9. Bacteria start growing that use Nitrite.
  10. Nitrite level starts to drop as Nitrate level slowly rises (Much less Toxic)
  11. Aquarist (that’s you) keeps Nitrate levels in check by doing weekly, partial water changes.
  12. Aquarist enjoys years of happy fish keeping and goes on to win the Nobel Prize.

Well, you never know… The point of this is, that the “break in cycle” takes some time to get going. A new tank isn’t ready for a full load of fish because the Bio-Filter hasn’t built up yet. Go slowly. Good things come to those who wait.

Had enough? Up for more? We can get more technical…ok then.

The types of bacteria that go through this Nitrogen Cycle are known as Aerobic Bacteria. Simply put, this means that they use Oxygen in their daily lives to do all the wonderful things they do. Without Oxygen, they cannot live. (Sound familiar?). They grow on surfaces in the aquarium. On the walls, gravel, decor, filter materials etc.

The best way to get these Aerobic bacteria to grow is to give them plenty of oxygen by having well aerated water AND to give them plenty of places to grow.

For many years, it was thought that Nitrosomonas and Nitrobacter were the primary culprits in this dance of waste management in the aquarium. Recent research by Dr. Tim Hovanec of Aquaria/Marineland in California has shown that we may have been mistaken for all these years.

Now there’s lots more that can be said on this subject, but I am tired and I think my Pizza might be here. Also do some web searching. There are many great resources out there. If you really want to rock your world, go out and get “Ecology of the Planted Aquarium” by Diana Walstad. Then take everything I just said and turn it inside out!

Well, not really, but you’ll see.

Now go get some fish![su_divider size=”1″]