Jamesc's Estimative Index Article

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George Farmer

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Jul 6, 2003
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Stamford, Lincs, UK
Estimative Index Explained

What is it?

Estimative Index is 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.

What do I need to buy?

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.

In the UK you are able to buy everything you need from Aqua Essentials and in the US from Greg Watson.

How do I use the chemicals?

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 making solutions then this is a guide to the amounts required.

Potassium Nitrate - 40g to 500ml of water and adding 10ml per 100L of water would give you a value of 5ppm.
Potassium Phosphate - 15g to 500ml of water and adding 5ml per 100L of water would give you a value of 1ppm.
Potassium Sulphate - 55g to 500ml of water and adding 10ml per 100L of water would give you a value of 5ppm
Magnesium Sulphate - 70g to 500ml of water and add 50ml once a week per 100L of water - this would give a target of 7ppm.
Trace mix - 15g to 250ml of water will give you about the same concentration as the ‘off the shelf’ products. It is also a good idea, if you can, to add 0.5ml Normal hydrochloric acid as this helps to prevent the chelator from breaking down. Some people complain of mould growing on top of the solution - keeping it in the fridge should help prevent this and prolong it’s life.

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 solution or 1/8 tsp KNO3 3x a week
5ml solution or 1/32 tsp KH2PO4 3x a week
2ml solution or 1/32 tsp K2SO4 3x a week
2ml or 1/32 tsp traces 3x a week

20-40 Gallons (76-152 litres)
20ml solution or 1/4 tsp KNO3 3x a week
12ml solution or 1/16 tsp KH2PO4 3x a week
5ml solution or 1/16 tsp K2SO4 3x a week
5ml or 1/16 tsp traces 3x a week

40-60 Gallons (152-227 litres)
30ml solution or 1/2 tsp KNO3 3x a week
18ml solution or 1/8 tsp KH2PO4 3x a week
8ml solution or 1/8 tsp K2SO4 3x a week
10ml or 1/8 tsp traces 3x a week

60-80 Gallons (227-303 litres)
40ml solution or 3/4 tsp KNO3 3x a week
25ml solution or 3/16 tsp KH2PO4 3x a week
11ml solution or 1/4 tsp K2SO4 3x a week
15ml or 1/4 tsp traces 3x a week

100-125 Gallons (380-473 litres)
70ml solution or 1 1/2 tsp KNO3 3x a week
40ml solution or 1/2 tsp KH2PO4 3x a week
16ml solution or 1/2 tsp K2SO4 3x a week
30ml 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 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.

A good weekly schedule to follow is as follows.

Sunday - 50% water change. Add KNO3, KH2PO4, K2SO4
Monday - Add traces
Tuesday - Add KNO3, KH2PO4, K2SO4
Wednesday - Add traces
Thursday - Add KNO3, KH2PO4, K2SO4
Friday - Add traces
Saturday - Rest day

As mentioned earlier potassium sulphate may not be required but adding it won’t do any harm.


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 CO2 levels.


Q. Why don’t you use test kits?
A. For 2 reasons. First hobby NO3 and PO4 test kits are so unreliable that unless you are prepared to calibrate them, the results cannot be trusted. Second there is no need. If you are doing a 50% weekly water change then the maximum amount you can build up is double your weekly dose. In practice it is a lot lower due to consumption by the plants.

Q. I’ve been told nitrates are harmful to fish.
A. This is based on organic nitrates and not inorganic nitrates that are dosed. There is no evidence to show that these nitrates are harmful within the limits of EI dosing. Many people keep shrimp and discus and breed fish whilst dosing EI.

Q. Does running a UV steriliser remove nutrients?
A. No, nothing gets removed from the water. UV light can break the chelator bond in the trace mix, but so does bright light. Many people run UV with no problems and there is no real evidence to show otherwise.

Q. Why does my water go cloudy?
A. This can happen if you have hard water and are dosing the trace mix or Seachem Flourish. Switch to Tropica Plant Nutrition to see if there is an improvement. Also don’t dose iron and PO4 on the same day as iron phosphate is formed that precipitates out.

Q. I have high NO3 and PO4 in my tap water. Do I still need to dose?
A. Yes. Get a water report from your water board to confirm first. If high you can skip the first dosing but then dose the second and third times as normal.

Q. EI doesn’t work as I get algae
A. Chances are this is because your CO2 levels are either too low or are fluctuating too much. Could also be because of ammonia caused by too many fish, overfeeding or lack of maintenance. It is also advisable to follow any substrate disturbance by a water change.


Here's a good overview - http://www.aquaticplantcentral.com/forumap...uide.html?pp=10
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