Estimative Index was created By Tom Barr to be a simple method for dosing nutrients to planted tanks without the need for monitoring nutrient levels using test kits. Basically the aquarist doses the tank with a slight excess of nutrients throughout the week to prevent anything from running out and does a large water change at the end of the week to prevent anything from building up. In this way we can provide a close approximation or ‘Estimative Index’ of the nutrient levels during the week as we know what is being added and also what is removed in the water change. Estimative Index works best with high light systems and well planted tanks, but can be adapted for lower light systems by reducing the dosing.

The fertilisers are the Macro Nutrients – Nitrogen (N), Phosphorus (P), Potassium (K) and the Micro Nutrients (or trace elements) which include Tropica Plant Nutrition, Seachem Flourish, CSM+B and chelated trace mixes.

The process of dosing is fairly simple. Each day (less for lower light systems) the fertilisers are dosed according to general guidelines based on tank size. At the end of the week a 50% water change is performed to reset the nutrient load in the tank. Then the process is restarted again. The 50% water change is just a guideline and doesn’t have to be followed but doing a larger water change will have greater accuracy of nutrient levels.

Potassium Nitrate
Potassium Phosphate (monobasic)
Traces – these can either chelated trace mix (CSM+B) or a commercial product such as Tropica Plant Nutrition or Seachem Flourish.

Also possibly required depending on your water supply.

Potassium Sulphate (Sulphate of Potash)– normally enough potassium is added with the potassium nitrate and potassium phosphate dosing. Some people like to add a little bit more.
Magnesium Sulphate (Epsom Salts) – added if you tap water is low in magnesium.
GH Booster or Seachem Equilibrium – If you have very low GH, ie. less than 3, then this will help by adding calcium and magnesium along with some potassium.

In the UK you are able to buy everything you need from or Fluidsensor Online and in the US from AquariumFertilizer. If you live in Canada then Rex Grigg will ship there.

There are two ways in which you can dose the nutrients, by making stock solutions or by adding dry powders using a set of standard spoons. To make up the solutions use the amounts shown below. If need be these amounts can be modified by using the Dosing Calculator to suit your particular needs.

Macro Solution
33g Potassium Nitrate
7.2g Potassium Phosphate
250ml Water

Trace Mix Solution
10g Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu)
250 ml Water
0.5 ml Normal Hydrochloric Acid

nb. It is preferable to make up the solutions in RO water or deionised water, but tap water can be used if need be. The hydrochloric acid is added to help preserve the solution and isn't absolutely necessary if you don't have it.

Using Solutions

Using the daily schedule below add the following 3 times a week:
5ml of Macro solution per 50 litres of water
2.5ml of Trace solution per 50 litres of water

Using Dry Powders

Use the tank volume guide below to determine amount to use. Amounts are added 3 times a week.
To convert to UK gallons multiply the litre figure by 0.22

40-80 litres
1/8 tsp KNO3
1/32 tsp KH2PO4
1/32 tsp traces

80-150 litres
1/4 tsp KNO3
1/16 tsp KH2PO4
1/16 tsp traces

150-225 litres
1/2 tsp KNO3
1/8 tsp KH2PO4
1/8 tsp traces

225-350 litres
3/4 tsp KNO3
3/16 tsp KH2PO4
1/4 tsp traces

350-500 litres
1 1/2 tsp KNO3
1/2 tsp KH2PO4
1/2 tsp traces

These figures give you a general idea of the amount of dosing required for a given tank size that is fully stocked with high lighting (approx 2 to 3 wpg). If you have very high lighting and are at the top end of the tank size bracket for your tank, it may be an idea to dose at the next tank up size bracket. Also if your tank is only 50% stocked then the amounts should also be reduced to 50%.

Daily Schedule

Estimative Index isn’t about aiming for parameters, but supplying everything to a slight excess of the plants requirements. These are general parameters that plants grow well in and as long as they are reached you shouldn’t have any problems.

Nitrate 20–30 ppm
Phosphate 1-3 ppm
Potassium 20-30ppm
CO2 30ppm

Estimative Index is highly dependant on a good level of CO2 which must be maintained at a stable 30ppm through the whole of the photo period. More than 90% of all algae problems on planted tank forums are because of poor or fluctuating CO2 levels.

Also of great importance, and often overlooked, is to have good water circulation around the whole tank. This is vital in getting the CO2 and nutrients to the plants. As a general guideline aim for 10x or more of rated filter flow to tank volume.

If you are dosing a leaner Estimative Index then you may wish to supplement the potassium levels to compensate for the lower amounts being added from the potassium nitrate and potassium phosphate. Adding an extra 10ppm potassium from potassium sulphate weekly should be fine. Use the Dosing Calculator to work out how much you require for your tank size.

If you have very low magnesium levels in your tap water then adding an extra 5-10ppm magnesium from magnesium sulphate at each water change should work well. Use the Dosing Calculator to work out how much you require for your tank size.

The Barr Report

UKAPS - The Estimative Index (EI) Dosing with Dry Salts