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 Aqua Essentials 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 a stock solution or by adding dry using a set of standard spoons. If you prefer making the solutions then below is a guide to the amounts required for the dosing guide. These amounts can be modified by using the Dosing Calculator to suit your particular needs.

Potassium Nitrate (KNO3) solution - 40g to 500ml of water.
Potassium Phosphate (KH2PO4) solution - 15g to 500ml of water.

Trace mix solution
15g Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu)
250 ml distilled water
0.5 ml Normal Hydrochloric Acid

It is possible to add the solutions into one bottle except for the trace mix as, depending on the chelator used, the iron could react with the phosphate. Generally speaking it is a good idea to make the solutions up in separate bottles until you are comfortable with the dosing.

nb. It is preferable to make up the solutions in RO water or deionised water, but tap water can be used if need be.

This is a general dosing guide that can be used with the following tank sizes. Solution volumes are calculated on making the stock solutions as above. Tank volumes are in US gallons. To convert to UK gallons multiply these figures by 0.83

10-20 Gallons (38-76 litres)
10ml KNO3 solution or 1/8 tsp KNO3 3x a week
5ml KH2PO4 solution or 1/32 tsp KH2PO4 3x a week
2ml trace solution or 1/32 tsp traces 3x a week

20-40 Gallons (76-152 litres)
20ml KNO3 solution or 1/4 tsp KNO3 3x a week
12ml KH2PO4 solution or 1/16 tsp KH2PO4 3x a week
5ml trace solution or 1/16 tsp traces 3x a week

40-60 Gallons (152-227 litres)
30ml KNO3 solution or 1/2 tsp KNO3 3x a week
18ml KH2PO4 solution or 1/8 tsp KH2PO4 3x a week
10ml trace solution or 1/8 tsp traces 3x a week

60-80 Gallons (227-303 litres)
40ml KNO3 solution or 3/4 tsp KNO3 3x a week
25ml KH2PO4 solution or 3/16 tsp KH2PO4 3x a week
15ml trace solution or 1/4 tsp traces 3x a week

100-125 Gallons (380-473 litres)
70ml KNO3 solution or 1 1/2 tsp KNO3 3x a week
40ml KH2PO4 solution or 1/2 tsp KH2PO4 3x a week
30ml trace solution or 1/2 tsp traces 3x a week

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 2.5 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%.

A good weekly schedule to follow is as follows:

Estimative Index isn’t really about aiming for parameters but supply 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.

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 5ppm 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