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Beginner's Guide To Fertilizers

PostPosted: Sat Nov 29, 2008 10:55 am
by Crazygar
A Beginner’s Guide to Fertilizers
and Basic KH/GH/CO2 Chemistry


These are the basic foundation for any plant and are required in significant quantities for growth. They are Hydrogen, Oxygen, Carbon, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur. Varying quantities of these will come from tap water and some will also come from fish wastes. If carbon is provided from the injection of carbon dioxide gas the demands for other nutrients will be higher. The big three of those listed are N-P-K, or Nitrogen, Phosphorus, and Potassium, with nitrogen generally considered to be the most important. Supplementation of N-P-K may be needed, depending on factors such as lighting, plant mass, etc. and can be supplied through regular fertilizing. Calcium and Magnesium may not need to be added, depending on the frequency and size of water changes and the level of hardness in the water you’re adding to the tank.


These are nutrients that are required in smaller quantities by plants. Examples are Boron, Chlorine, Cobalt, Copper, Iron, Manganese, Molybdenum, and Zinc. Popular choices of adding micronutrients are dry mixes such as CSM+B Plantex or liquid products like Flourish or Tropica Plant Nutrition Liquid, formerly known as Master Grow.

Target Nutrient Levels

The following are general suggestions regarding the level of nutrients in your planted tank. Ideal levels will vary so an ideal amount cannot be given and the below ranges are a rough guideline. These ranges are more applicable to tanks with moderate to high lighting and CO2 injection.

CO2: 25-35 ppm
Nitrates: 5-25 ppm
Phosphates: 0.5-2.5 ppm
Potassium: 10-25 ppm
Iron: 0.1-0.2 ppm
Calcium: 10-35 ppm
Magnesium: 2-6 ppm
GH: 3 degrees or 50 ppm and up
Two Methods of Dosing – EI and PPS

EI stands for Estimative Index. It was developed by Tom Barr. For CO2 injected tanks, the EI system uses larger doses of nutrients about every other day and large water changes on a weekly basis. Water changes are in the area of 50%. Trace elements and macronutrients are dosed on alternating days.

PPS stands for Perpetual Preservation System. It uses small amounts dosed from four pre-mixed solutions in low quantities daily - only a few milliliters for most tanks. You will need a scale capable of measuring grams and a minimum of Nitrate and Phosphate test kits to use this system. PPS doesn’t call for large water changes like EI but can accommodate for them. This system of fertilizing is less well known than EI.

Organic vs. Inorganic Nutrients

Organics are defined as matter that was living or came from a living organism. This could be fish waste, dying plants that are breaking down, or even decaying food. Nitrogen, especially in the form of ammonia, and phosphorus are two of the primary organics to be concerned about in a planted aquarium. A build up of organics can contribute to difficulties with green water, cyanobacteria, and other forms of algae in moderate to high light tanks. Effective methods of reducing dissolved organics are water changes, gravel vacuuming, and filter cleaning.

The nutrients that we add via KNO3, K2SO4, etc. are inorganic. Inorganic nutrients can be immediately taken in by plants while organic nutrients must be converted before they can be used. This conversion process is known as mineralization.

GH – General Hardness

GH is given in degrees or ppm and represents a total of the Calcium and Magnesium present in the tested water. It is the GH that is relevant when considering the water hardness of fishes’ natural habitats. African cichlids come from hard water, therefore a higher GH, while most South American fish come from softer water, therefore a lower GH. Nearly all common aquatic plants will do very well in harder water but some demanding types such as Tonina and Eriocaulon species require soft water and prefer an acidic substrate.

KH – Carbonate Hardness

KH is a measurement primarily made up of carbonates. KH indicates the amount of acid the water can handle before reaching a certain pH, also known as its capacity to buffer water. It can be stated in degrees or ppm. To convert ppm to degrees divide by 17.86. The measurement of KH in degrees is used with the recording for pH to find approximate CO2 levels in an aquarium.

CO2 – Carbon Dioxide

Maintaining adequate levels of CO2 have shown to benefit plant growth and reduce algae problems as long as the amounts of other nutrients are provided with an appropriate balance. If injecting CO2, a minimum level of 20-25 ppm is suggested with 30-35 ppm being more ideal. Diffusing carbon dioxide into an aquarium will lower pH levels. In a planted tank, CO2 levels will decrease and therefore pH will rise during the day. At night, the plants will release CO2 and therefore lower pH.

The following will give concentrations of approximately 30 ppm of CO2:
pH of 6.4 and KH of 2.5 degrees
pH of 6.8 and KH of 6 degrees
pH of 7.2 and KH of 16 degrees
Fertilizer Chemicals, Dosages and Changes to Water Chemistry

The below totals are examples that fall within the above mentioned target levels. For more accurate dosages, grams should be used instead of teaspoons. A digital scale will be needed for this. However, teaspoons provide an approximate value that is generally close enough for most hobbyists. The below figures assume the chemical is added directly to the tank and not diluted in a solution. Another option would be to mix pre-determined solutions. For example, dissolve 15 grams of KNO3 in 500 mL of distilled water; 10 mL of this solution will add approximately 2.5 ppm of NO3 in 20 gallons of water or 1 ppm of NO3 in 50 gallons.

KNO3 - potassium nitrate
¼ teaspoon equals approximately 10 ppm of NO3 and 6.6 ppm of K+ in 20 gallons of water
¾ teaspoon equals approximately 13 ppm of NO3 and 8 ppm of K+ in 50 gallons of water
KH2PO4 - mono potassium phosphate
0.02 tsp. or 0.11 grams equals approx. 1 ppm of PO4 in 20 gallons of water
0.05 tsp. or 0.28 grams equals approx. 1 ppm of PO4 in 50 gallons of water
K2SO4 - potassium sulfate
¼ tsp. equals approximately 10 ppm of K+ in 20 gallons of water
¾ tsp. equals approximately 11 ppm of K+ in 50 gallons of water
KCl – potassium chloride
¼ tsp. equals approximately 8-10 ppm of K+ in 20 gallons of water
½ tsp. equals approximately 6-8 ppm of K+ in 50 gallons of water
MgSO4.7H20 - magnesium sulfate, aka Epsom salts (will raise GH)
½ tsp. equals approximately 3.1 ppm of Mg in 20 gallons of water
1 tsp. equals approximately 2.5 ppm of Mg in 50 gallons of water
CaSO4 - calcium sulfate (will raise GH)
1 tsp. equals approximately 11.3 ppm of Ca in 20 gallons of water
2 tsp. equals approximately 9 ppm of Ca in 50 gallons of water
CaCl2*1 - calcium chloride (will raise GH)
½ tsp. equals approximately 10.5 ppm of Ca in 20 gallons of water
1 tsp. equals approximately 8 ppm of Ca in 50 gallons of water
CaCO3*2 - calcium carbonate*3 (will raise GH, KH and pH)
1 tsp. equals approximately 6.35 ppm of Ca in 20 gallons of water
3 tsp. equals approximately 7.6 ppm of Ca in 50 gallons of water
NaHCO3*2 - sodium bicarbonate, aka baking soda (will raise KH and pH)
1 tsp. raises the KH of 50 litres of water, or 13.2 gallons, by approximately 4 degrees
1Note there are weaker versions that will not add as much Ca to the water. (CaCl2.2H20 and CaCl2.6H20)

2It is strongly recommended that you do not use calcium carbonate and/or baking soda without doing proper research. They are not for beginners and are only included in this article for the sake of completeness.

3Calcium carbonate is also found naturally in seashells, corals, snail shells, and rocks such as marble, calcite, and limestone.

Two excellent resources are Chuck Gadd’s downloadable nutrient and CO2 calculator program and Aquatic Plant Central’s Fertilator, an online Macromedia Flash based nutrient calculator. Both were used to find the values listed above.

Glossary to Terms Used Above

NO3 = nitrate
PO4 = phosphate
K+ = potassium
Ca = calcium
Mg = magnesium

Written by Christian Cox (Captain Hook)