Science of Freshwater Aquariums (part II)

After discussing starter equipment, we will discuss nitrogen cycle - most important segment of aquarium ecosystem.

Nitrogen Cycle

Nitrogen cycle

Nitrogen Cycle begins with decomposing of organic matter. This includes uneaten food, plant parts and fish feces. When it decomposes it results in ammonia which is highly toxic to fish in aquarium. Smaller part of ammonia is used by plants and greater part is used by Nitrosomonas (Nitrosomas) to produce nitrites. Since Nitrites are still highly toxic it needs to be converted yet again. Nitrospira converts dangerous nitrites into less toxic nitrates. Nitrates are used by plants and are removed from aquarium by water changes.

Step-by.step cycle

Aquarium Cycling

When you set up your new aquarium it is required to cycle it. Cycling means allowing time for bacteria colonies to form in aquarium which help dissolve organic waste into less toxic compounds. This process is usually done without any animal presence (because of high toxicity), however some fish keepers cycle with few fishes that are more resistant to ammonia/nitrites/nitrates.

These are the steps you should take to cycle your aquarium:

1) Ammonia - To start the cycling process first you need ammonia. You can "produce" some by putting some fish food in aquarium and let it decompose.

2) Oxygen - Since Nitrosomonas are aerobic bacteria, they require as much oxygen as possible. This can be done by letting the water pump agitate surface of water and by adding oxygen stones in aquarium.

3) Light - These bacteria love light and if there are no fish in aquarium, light should be kept on all the time.

4) Temperature - Higher temperatures allow bacteria to grow even faster. If there are no fish in aquarium temperature should be kept around 28C (83F).

5) Filter - In order to accumulate bacteria in filter material, filter has to be running all the time.

6) Plants - Plants reduce nitrate and ammonia levels and produce oxygen for Nitrosomonas.

7) Starters - There are bacterial colony starters available in liquid form. These are usually dropped on filter to make starter colonies.


After cycling has started, three phases can be expected:

Cycling of a new tank

1) Ammonia spike - after few days Nitrosomonas will start appearing. This is so called ammonia spike and from that point on, ammonia levels will go down and nitrite levels will go up. Shortly afterwards Nitrospira will appear and start producing nitrate.

2) Nitrite spike - when ammonia levels hit 0, nitrite level will spike. At this point nitrate level will start going down and nitrate level will continue to grow even faster.

3) Nitrite 0 - at one point nitrate levels will go down to 0. This shows that the cycle is complete. Due to high nitrate level around 40% of water should be changed.

After few more days of measuring ammonia/nitrite/nitrate levels and making sure ammonia and nitrite are 0, it is safe to introduce fish into aquarium.

Oxygen Cycle

Finally there is oxygen cycle. This is a very small part of ecosystem balance. Fish need oxygen (O2) dissolved in water in order to survive. This is accomplished by agitation of water surface (with pump), oxygen stones and plant oxygen production (photosynthesis). Animals and plants also produce carbon dioxide (CO2). This is reused by plants for photosynthesis or it exits ecosystem via surface agitation.

Keep in mind:

• more animals means less oxygen and more carbon dioxide
• less light in planted aquarium means less oxygen and more carbon dioxide

Bear in mind that some fish need to go to surface and take oxygen (Anabantoidei - Labyrinth fish) like bettas and gouramis, so it is wise to keep the surface area accessible for those fish.

Science of Freshwater Aquariums (part I)

When I decided to buy aquarium for the first time, I was pretty lost. I didn't know what to buy and why do I need it. In order to help future aquarium owners I will create my first post on one of my hobbies - aquaristics.

First off - what are the basic items needed for freshwater aquariums?

1) Aquarium - It is generally not recommended to buy bowls since it distorts fish view and is considered to be painful for fish. Bigger aquariums are usually better (although some fish species prefer smaller environments). If you opt for custom made aquarium, keep in mind that thickness of glass depends on volume of aquarium.

2) Lid with lights - In order to minimise vaporisation as well as protect ecosystem from outside, lid is needed to cover the aquarium. Lights are crucial because it provides fishes vitamin D. Different types of light are beneficial for different things - some are good for fish colouration, some are good for plant growth.

3) Filter - Fish don't always eat all of their food and they defecate in the aquarium. This produces deadly ammonia. Ammonia needs to be converted to nitrite by bacteria. Nitrite is still toxic for fish so it has to be further converted to nitrate. Nitrate is not toxic in smaller quantities and can serve as growing supplement for plants. In order to keep aquarium clean filter collects decaying matter and houses bacteria which convert it to nitrates (as final product).

4) Pump - Even though fish live in water they still need oxygen to survive. Without oxygen dissolved in water fish will suffocate. Pump has dual role - first is to stir water so more oxygen can be dissolved in water and second is to drive water through filter so it can be cleansed.

5) Heater - Not all species live in cold waters, some (e.g. Guppy) live in tropical waters. To provide stable temperature at all time is important for fish health as sudden temperature changes can lead to stress, sickness and death.

6) Thermometer - To make sure you have constant temperature in aquarium you need a thermometer.

7) Siphon - Some decay can be stuck among gravel at the bottom of the aquarium. It can be removed with a simple siphon that circulates the gravel and sucks up decaying matter.

8) Water conditioner - Tap water contains chlorine and heavy metals. This is harmful to aquarium organisms and should be removed with conditioner.

9) Bacteria starter - If you are setting up your aquarium for first time, these bacteria needs to be introduced to aquarium in order to multiply and start converting ammonia to nitrate.

10) Gravel or sand - Bottom dwellers prefer sand due to their body touching the bottom which is not as harsh as gravel.

11) Fish food - Be sure you know what your fish eats. Some eat algae, some eat meat, some are bottom dwellers, some swim in mid/top section. Take all that in consideration when buying food.

12) Net - Safest and least stressful way of handling fish is with net. This way it does not come in contact with your hands (which might have some harmful bacteria/fungi on them). For fish with delicate fins use finer nets.

13) Plants and decorations - Plants can greatly improve your ecosystem by consuming nitrate and producing oxygen. They also serve as hiding and resting places for fish. When putting decorations make sure they don't have sharp edges and that they are properly treated before introducing it to aquarium.

That is all you need to start your aquarium.