Nutrients (NH4+, NO3-, K+)

Beyond the "building blocks of life" provided by water and CO2 (oxygen, hydrogen and carbon), two other important nutrients are required: nitrogen and potassium. 

The nitrogen cycle

Nitrogen (N) compounds can be released by the death of organisms, urine of fish, and algae. These compounds are processed by decomposing bacteria which created ammonia gas (NH3) and other products.

Ammonia  When the ammonia gas contacts water, it forms ammonium hydroxide (NH₄OH) which is highly toxic substance but the ammonium part (NH4+) can be used by plants. Ammonia is very dangerous for fish (depending on temperature, pH and water salinity). If the pH is low (more acid), more ammonium hydroxide is neutralized than in water with a higher pH. (more alkaline) Ammonium hydroxide is consumed by the Nitrosomonas bacteria, which in the presence of oxygen transform Ammonia into nitrite (NO2-)
2NH3 + 3O2 -> 2HNO2 + 2 H2O + energy. 

The HNO2 (nitrous acid) dissolves in water, releasing the nitrite ion (NO₂^-) which is also highly toxic. The Nitrobacter bacteria transforms nitrite in Nitrate NO3- :2 HNO2 + 02 -> 2 HNO3 + energy. 

Nitrate (NO3-), just like ammonium (NH4+), is used by plants but too much nitrate can cause excessive algae growth.

Maximum levels :

	Ammonia/Ammonium (NH3/NH4+) 0.5 ppm.
	Nitrite (NO2-) 0.3 mg/l.
	Nitrate (NO3-) 50 mg/l.

Potassium

Potassium (K+) is also usually available from fish food. Unfortunately, potassium is difficult to measure in the water. If there are enough nitrates, there is usually enough potassium. Some fertilizers contain additional potassium and can be used to be on the safe side.

Some plants can concentrate carbon, potassium, nitrogen, phosphorus, iron or the lesser trace elements and store it for later use. This means that plants may do well for a while, using stored nutrients, and then mysteriously wither if they can't replenish their supply. This also means that some plants may "out-compete" others for required nutrients, preventing the other plants from doing well.

Regular water changes are an important part of keeping a planted aquarium healthy since many of the nutrients and trace elements are in tap water. Changing 25 percent every two weeks is recommended.

The following table is based on data from the Feb, 1988 "Today's Aquarium, the International Magazine of the Optimum Aquarium", ("Aquarium Heute" in German), published by Aquadocumenta Verlag GmbH.

      Average nutrient content of plants and aquarium water  

+-----------------------------------------------------------------+
| Symbol  Nutrient        Plant     Water    Absorbed as    Concen|
|                         mg/kg      mg/l                   Factor|
+-----------------------------------------------------------------+
| O       Oxygen         48,000    880,000   H2O             0.02 |
|   Abundantly available in the water                             |
|                                                                 |
| C       Carbon         36,000     Varies   CO2(HCO3-)      1000 |
|   Absent if no CO2 injection                                    |
|                                                                 |
| H       Hydrogen        6,000    110,000   H2O             0.02 |
|   Abundantly available in the water                             |
|                                                                 |
| K       Potassium       3,600          5   K+              1000 |
|   Sufficient with good feeding, otherwise fertilizing           |
|                                                                 |
| N       Nitrogen        3,200          5   NH4+/NO3-       1000 |
|   Too much nitrate with good fish feeding                       |
|                                                                 |
| S       Sulphur           660         15   SO4--             50 |
|   Source: fish food and mains water                             |
|                                                                 |
| Ca      Calcium           650         90   Ca++              10 |
|   Absent in soft water                                          |
|                                                                 |
| P       Phosphorus        460        0.1   HPO4--/H2PO4-   1000 |
|   Too many phosphates with good fish feeding                    |
|                                                                 |
| Mg      Magnesium         210         18   Mg++              10 |
|   Absent in soft water                                          |
|                                                                 |
| Fe      Iron               15          0   Fe++/Fe+++      1000 |
|   Absent under good light, unless fertilized                    |
|                                                                 |
| Other   Trace elements     10          0   Ions            1000 |
|   Sufficient with good feeding, otherwise fertilizer            |
+-----------------------------------------------------------------+

Notes: "mg/kg" and "mg/l" are roughly parts per million or "ppm"

"Concen Factor" is how much plants can store beyond their needs for growth, i.e., plants can store 1000 times more iron than they need.

Also:

• Tilapia Topic, Aquaculture Environment, Nitrogen

See also:

• http://www.thekrib.com/Plants/CO2/co2-pvc.html Low cost CO₂ sensor an control system.+

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