🐠 The Nitrogen Cycle in a Marine Aquarium
A hobbyists guide from Essex Marine Aquatics.
Think of the nitrogen cycle as the aquarium's natural waste management system. When you feed your fish and they poop (or uneaten food rots), waste gets released into the water. That waste contains ammonia (NH₃) – and ammonia is toxic to your fish.
Luckily, helpful bacteria take care of this – but they need time to establish. Here’s how the cycle works:
🌀 Step-by-step Breakdown:
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Ammonia (NH₃) – The Bad Stuff
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Comes from fish waste, leftover food, and dying plants.
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Very toxic to fish, even at low levels.
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Nitrite (NO₂⁻) – Still Bad
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After a while, Nitrosomonas bacteria grow in your filter and rocks.
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They convert ammonia into nitrite.
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Still toxic, but a step in the right direction.
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Nitrate (NO₃⁻) – Less Harmful
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Next, Nitrobacter (or similar) bacteria convert nitrite into nitrate.
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Nitrate is much less toxic, but still needs to be controlled.
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Export or Use of Nitrate
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In a reef tank, corals and macroalgae can use some nitrate.
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You still need to remove excess nitrate through:
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Regular water changes
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Using a protein skimmer
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Adding live rock, refugiums, or nitrate-removing media
💡 Why This Matters to You
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A cycled tank has all the bacteria in place to handle waste.
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You must let the tank cycle fully before adding fish – this usually takes 4–6 weeks. With the exception of using instant cycle products, however, you must add livestock very slowly using these products, the aquarium will still increase its capacity to support more life over time, your biological filter will not instantly be running at its possible peak performance.
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Test your water regularly for ammonia, nitrite, and nitrate.
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If ammonia or nitrite is present, don’t add more fish!
If ammonia or nitrite is present, do look at options to remove or reduce levels!
This is one in a series of articles to help you understand the foundations you need to set up and maintain a successful marine aquarium.
