Next to a suitable substrate, light, CO2 and pH are far more important than any fertilizer.
Light is needed for plants to produce energy by photosynthesis while CO2 helps plants utilize the minerals required.
Plants will not be able to utilize nutrients, minerals, or light at low CO2 levels.
Low light and CO2 levels can not be compensated for by adding fertilizers, as the added nutrients will not be utilized by the plants.
Using light and Co2, the plant creates carbohydrates and oxygen (photosynthesis) while the pH level determines the solubility of the nutrients. A pH above 7.3 will cause most plants to lose their ability to utilize many essential nutrients. Next to that, most minerals precipitate out of solution and can no longer be absorbed by the plants. The pH should therefore be kept at 6.5 - 7.0 for optimum plant growth.
Before adding any fertilizer the CO2, light, and pH levels have to be checked and adjusted accordingly.
As a general rule, the lighter the green of the leaves, the greater the rate of oxygenation, the faster the growth rate, and the higher the requirement for nutrients.
The composition of nutrients is conveniently divided into macro and micro nutrients. Macro nutrients are needed in larger quantities while micro nutrients are sufficient in smaller quantities.
Macro nutrients are calcium, sulfates, phosphates, potassium, chloride, sodium, nitrogen and magnesium. These nutrients are provided by fish and fish food in ample supply. Macro nutrients do not need to be added frequently, if at all, as they will be mostly replenished through water changes.
Essential micro nutrients such as iron, manganese, zinc, boron, copper, cobalt, and molybdenum on the other hand have to be added frequently. The main function of these nutrients is the promotion of growth hormones, photosynthesis, cell development, plant metabolism, and nitrogen assimilation.
The assumption that plants take on most nutrients through their leaves is incorrect. Leafs absorb CO2 and release oxygen. Essential nutrients such as iron, phosphates and nitrogen are readily absorbed by the roots under anoxic conditions found in the substrate.
Plant fertilizers are available as liquid or substrate fertilizers. Both should only contain the micro nutrients. Liquid fertilizers have to be dosed more frequently; substrate fertilizers will last longer. Since there are no obvious differences in efficiency, it is up to the aquarists' preference which to use.
Next to the micro nutrients, fertilizers contain chelates. The chelate is an organic molecule which binds metal ions thus protecting them from early precipitation. The preferred type is abbreviated DTPA because of its stability up to a pH level of 7.5
Unfortunately some fertilizers contain the chelate EDTA, which is much cheaper. However chelate EDTA is only stable at a pH up to 6.0 and therefore mostly useless in aquariums.
Another important yet often overlooked aspect in using fertilizers is water conditioners. Many conditioners eliminate heavy metals and since many micro nutrients are metals, plants can be deprived of essential nutrients despite the frequent addition.
Fertilizers can be made at home or bought commercially. The home version is known as "poor mans dupla drops" (PMDD)* relying on the following ingredients:
2 Teaspoons potassium sulfate
1 Teaspoon potassium nitrate
2.5 Tablespoon hydrated magnesium sulfate
1 Tablespoon chelated trace elements mix
The trace element mix should preferably contain DTPA chelated Fe (7%) B (1.3%) Mn (2%) Mo (0.06% Zn (0.4%) Cu (0.1%)**
Mixed with 1.5 cups of distilled water, the mixture can be stored in the refrigerator.
The required dosage varies depending on plant quantity, growth levels and aquarium size. The correct amount has to be determined by trial and error i.e. starting with a few drops per 10 Gallon while monitoring the iron and nitrate concentration. Iron should be around 0.1 ppm; lower levels also indicate the depletion of the other trace elements. Nitrates should be kept around 5 ppm. Nutrient deficiencies are described in detail in our plant health guide.
Adding fertilizers to promote aquatic plant growth should only be considered in medium to heavily planted aquariums. A few plants should do fine without.
* Ingredients can be obtained at chemical supply stores, trace element mixes are often available in aquatic stores.
** Fe (Iron) B (Boron) Mn (Manganese) Mo (Molybdenum) Zn (Zinc) Cu (Copper)