Unlimited Waterplant Growth

In waterplants, unlimited growth is achieved when ample amounts of fertilizer are added and the phosphate supply is also not limited, in contrast to the limited-supply approach. The Estimative Index is a fertilizer concept that strives for unlimited growth.

The Estimative Index

In a nutshell, the Estimative Index method safeguards a permanent nutrient supply of the plants at all times. This is achieved by ample fertilizer dosages. The nutrients are kept on a constant level by a 50% waterchange once a week. There is no need to test the water when you apply this method as the waterchanges prevent the nutrients from accumulating and thus protect the tank from possibly toxic nutrient concentrations.

This method is based on tests carried out by Tom Barr. First he established the maximum uptake rate of nutrients, the experiments on this subject were carried out under a very high light output (450 micromol/m2/sec at an 8-cm-distance from the light source) and high CO2 levels, over a period of approximately 3 weeks. The parameters were taken at intervals of 24 hours and give the following values for the plants? daily nutrient uptake:

Nutrient Consumption
NO3 1-4 mg/l
NH4 0,1-0,6 mg/l
PO4 0,2-0,6 mg/l

This is the maximum absorption rate; adding less nutrients does not necessarily mean that the plants will develop deficiency symptoms. When more nutrients are provided, the plants do not react with a still-increasing growth rate, and an extreme nutrient concentration may even result in problems for fauna and flora.

The maximum daily absorbtion rate suggests the following nutrient supply rate:

Nutrient Supply
CO2 25-35 mg/l
NO3 5-30 mg/l
K 10-30 mg/l
PO4 1,0-3,0 mg/l
Fe 0,1-0,5 mg/l

This nutrient list gives us a relatively high PO4 value which makes for the main difference between this fertilizing system and others. The Estimative Index basically assumes that a constantly high nutrient supply does not trigger algae but that a lack of nutrients and, in its wake, poor plant growth, is responsible for an algae outbreak. According to Tom Barr, a plentiful nitrogen fertilization with ammonium (NH4) alone, lots of light and the addition of CO2 will lead to an algal bloom relatively fast. This problem can be remedied by using nitrate-based fertilizers instead of ammonium-based ones for nitrogen supply. He especially refers to a study by Bachmann et al., which did not find any link between algal growth and phosphate supply as long as there was a sufficient amount of waterplants. The study also deals with nutrient accumulation in Floridian lakes and its consequences for plant and algal growth. Especially Floridian waters with their tropical temperatures and their mostly clear, shallow and well-vegetated lakes compare well with our aquariums, and according to Tom Barr they provide better insights for our specific subject than limnological examinations of coldwater lakes.

The Estimative Index prescribes the following daily dosage in order to reach and to maintain the target values given above:

Nutrient Target
NO3 4,6 mg/l
K 3,25 mg/l
PO4 0,9 mg/l
Fe 0,5-1 ml Mikro Basic Eisen auf 40 l

Fertilization with Aqua Rebell products might look as follows:

5 ml of Makro Basic Estimative Index per 40 litres per day
0.5-1 ml of Mikro Basic Eisen (complete iron fertilizer) per 40 litres per day

"Complete Iron Fertilizer" means a fertilizer containing all essential micronutrients for plants, not only iron. Only the combination of iron and micronutrients gives plants everything they need.

The classic Estimative Index method adds the nutrients one day around the other. One day, double the daily dosage of macronutrients is given, and the next day, double the amount of micronutrients. This is done three times a week, with a one-day break. The following table illustrates the course of the week:

Day Macronutrients Micronutrients
Monday - 4 ml Mikro Basic - Eisenvolldünger
Tuesday 40 ml Makro Basic - Estimative Index -
Wednesday - 4 ml Mikro Basic - Eisenvolldünger
Thursday 40 ml Makro Basic - Estimative Index -
Friday - 4 ml Mikro Basic - Eisenvolldünger
Saturday - -
Sunday 50% water change
40 ml Makro Basic - Estimative Index

It is not necessary to dose the nutrients exactly, the only important thing is that there are sufficient quantities at any time, and the name of the method "Estimative Index" mirrors that. The amount of nutrients needed is just estimated.

With the Estimative Index, a permanent nutrient supply is guaranteed, and only the CO2 content of the tank is a limiting factor for plant growth. Thus, a relatively high and constant addition of CO2 is necessary for optimal plant growth if you fertilize your tank according to this method.


The Estimative Index should be adapted to the specific tank in order to safeguard that there are always suffient quantities of nutrients, however, the levels should not increase too much either. In the long run, too high a dosage is not only detrimental for some animals living in the aquarium, it is also very expensive. Thus it is recommenable to start with the maximum dosage and to lower it every two weeks until you notice that the plants grow more slowly or stop growing altogether. When this point has been reached, raise the dosage to the amount you were adding the fortnight before, and you have the optimum quantity for your tank. The Estimative Index is a method for well-lit tanks. Tanks with less light should be provided with less nutrients, in a moderately lit tank a normal daily dosis may be sufficient for an entire week.