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Experts in reptiles and aquatics

What is the Nitrogen Cycle in Freshwater Aquariums?

Having the idea of setting up an aquarium feels like a brilliant idea until you do a bit of digging on the Internet and begin to get bamboozled with the barrage of information you read. One of the first hurdles you'll come to is the ‘Nitrogen Cycle’, Here at Perry’s Pet Supplies we’re going to briefly describe the basics of the nitrogen cycle and the key points you'll need to take to set up a healthy and happy aquarium for your fish.

What is the Nitrogen Cycle?

The Nitrogen Cycle in your fish tank is a four step process of converting the dangerous waste products of your fish and aquarium (the ammonia) to the less dangerous gas of nitrate, which we must remove ourselves with a water change. This process is done mainly in your aquariums filter, whether it is an internal or external filter the process is exactly the same. The final stage is for us to remove excess nitrate by hand with a water change. We've broken down the cycle into steps below for easy reading and digestion.

Step One: Ammonia produced

Ammonia is the biological waste product of the fish and excess rotting food in your aquarium. This is very dangerous to your fish even at low levels and has the potential to be lethal. In a healthy aquarium you should expect to see a reading of zero for ammonia when testing. Excess ammonia is thought to be the number one killer of fish in aquariums, so we must get rid of it, on to step two.

Step Two: Ammonia to Nitrite

To remove the ammonia it needs to be converted to nitrite in your filter by what we call ‘Beneficial Bacteria’. This bacteria is on our side and lives in your filter sponges and media and converts all the ammonia that passes by it. Nitrite is still harmful to your fish however not to the extent of ammonia. That being said you should still have a zero reading for it in your aquarium when testing (see your water test kits instructions for more information). Next we must convert the ‘nitrite’ to the even less harmful ‘nitrate’, Step three.

Step Three: Nitrite to Nitrate

Now that we have successfully got rid of the ammonia we must now get rid of the nitrite. This is done exactly the same as step two in your filter. Even more of the ‘Beneficial Bacteria’ converts the nitrite to the less harmful gas of ‘nitrate’. Fish will accept a certain amount of the nitrate in their water but not excessive amounts as too much can also be dangerous. I would recommend keeping it below 10PPM(Part Per Million). To take over Mother Nature’s job and complete the cycle, this is where you come in. Step four.

Step Four: Nitrate to Water Change

The water change is the end of the cycle and can only be done by us the fish keepers. Once the Nitrate has reached a certain level we must remove it by hand, this is done with water changes. By taking out certain amounts of water we are removing the harmful gasses and pollutants which have built up over time. Adding new fresh water to you aquarium replenishes vital elements and minerals which your fish will love. It is recommended to change 25% of your aquariums water every week as a minimum. However this is only a guideline and everybody’s tanks are different. I personally do a 50% water change every week on my tropical community tank. Remember to add water conditioner every time you do a water change.


We appreciate that was a lot to take in and a lot of nitrogen like words. However keeping it short and sweep and only get the main need to know points across at the minute is all we need. Understanding the Nitrogen cycle is crucial to keeping your fish happy and healthy but also to help you diagnose any potential illnesses and problems caused by poor water quality.

See the below cycle for the most basic overview of what you'll need to know and how it works.

We hope you took what you needed from the short description; sometimes you don’t need to know the long latin scientific names to understand a simple process. More often than not the very basics will do to get you started.

Many thanks for reading.

Kind regards,

From Ryan & all at Perry’s Pet Supplies.


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