Estimative Index Fertilization Method

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John N.

AC Members
Feb 1, 2006
Estimative Index Fertilization Method

Let’s cut to the chase. People want to have a lush planted aquarium with as little work and money as possible. When it comes to fertilizing the aquarium finding an easy, cost-efficient way can be quite a predicament – but it doesn’t have to be.

The Estimative Index (EI) popularized by Tom Barr is a straightforward fertilization method for dosing nutrients in a planted aquarium without the need for monitoring water parameters. This method works on the basic principle of supplying more nutrients to plants then what they actually consume during a week’s timeframe. At the end of each week, the hobbyist “resets” the aquarium and nutrient levels by performing a large waterchange that flushes out the system. This whole process creates an “estimative” amount of nutrient levels that are more than adequate for plants to grow healthily.

The Estimative Index method works best with high light and heavily planted aquariums, but can work with lower light levels and less plant mass by reducing the frequency or amount doses in the suggested regimes. It assumes the aquarium will have adequate CO2 of 30 ppm or above. In both high light and low light situations, the hobbyist will dose fertilizers daily according the instructions below, and do a weekly 50% water change.

The primary fertilizers used in any planted aquarium are the macro nutrients – Nitrogen (N), Phosphorous (P), Potassium (K), and the micro/trace elements (Plantex CSM+B, Seachem Flourish, Tropica Plant Nutrition). Iron (Fe) can also be supplemented if desired, but in most cases not necessary.

How do I use these fertilizers and the Estimative Index?

Fertilizing via EI is simple. Every other day dose the prescribed macros elements, and on the off days add in the trace/micro elements. Perform a 50% waterchange at the end of the week. By following one of the commonly used dosing programs below for your specific tank size you can ensure your plants are getting what the need throughout the week.

10-20 Gallons
1/8 tsp KNO3 3x a week
1/32 tsp KH2PO4 3x a week
1/32 tsp K2SO4 3x a week
1/32 tsp (2ml) traces 3x a week

20-40 Gallons
1/4 tsp KNO3 3x a week
1/16 tsp KH2PO4 3x a week
1/16 tsp K2SO4 3x a week
1/16 tsp (5ml) traces 3x a week

40-60 Gallons
1/2 tsp KNO3 3x a week
1/8 tsp KH2PO4 3x a week
1/8 tsp K2SO4 3x a week
1/8 tsp (10 ml) traces 3x a week

60-80 Gallons
3/4 tsp KNO3 3x a week
3/16 tsp KH2PO4 3x a week
1/4 tsp K2SO4 3x a week
1/4 tsp (15ml) traces 3x a week

100-125 Gallons
1 1/2 tsp KNO3 3x a week
1/2 tsp KH2PO4 3x a week
1/2 tsp K2SO4 3x a week
1/2 tsp (30ml) traces 3x a week

Example of a dosing program for a 29 gallon tank.

*K2SO4 is not required for dosing unless you need the extra Potassium (K). This K is found in KN03 and KH2P04. Dosing these two according to above will yield sufficient K levels. Therefore, one will be fine dosing only KN03 and KH2P04, and Plantex. However, if one finds the need to supplement the K2SO4, there is no harm by dosing this chemical as part of your fertilization regime.

Where can I buy the fertilizers and chemicals? can provide you with the necessary chemicals for dry and liquid dosing of the above. For micro - trace elements, Plantex CSM+B, Seachem Flourish, and Tropica Plant Nutrition are equivalent to each other. For the Seachem and Tropica brands visit and

One pound bags of each of Aquarium Fertilizer Chemicals will last at least 1 year:

· (Trace) Plantex CSM+B
· (N) Potassium Nitrate KN03
· (P) Monopotassium Phosphate KH2P04
· (K) Potassium Sulphate K2S04 (optional)

Special Notes:

Providing optimal CO2 levels of at least 30 ppm are necessary for plants to prosper and out-compete algae. If algae issue arise, remove all visible algae and infected leaves. Recheck CO2 levels, and possibly reduce and adjust the lighting period.

Direct dry dosing into the tank is perfectly fine. Many dosing straight into the tank, or they dissolve each chemical in water before adding.

Making a Liquid Stock of Plantex CSM+B is more often mixed into a bulk liquid solution since some find it more convenient to dose their trace elements this way. The recipe for this solution is 1 tablespoon to 250ml water is equivalent to: 20 ml = 1/4 teaspoon of dry Plantex. This solution is stored in refrigerators to prevent mold from forming within the container. For making stock solutions for NPK use Chuck Gladd's Conversion Calculator.

Small dosing teaspoons (smidgen, dash, pinch) can be found at Linen & Things, Bed Bath and Beyond, Wal-Mart, dollar stores, eBay and other online retailers. To identify the specific measurements of your smidgen, dash, pinch set, a 1/8 tsp should fill a ¼ tsp in 2 tries, 1/16 tsp in 4 tries, and a 1/32 tsp in 8 tries.

You will see results right after the first week of dosing. Your plants will grow faster and your plants will look healthier. Keep with the dosing program, and you'll see your planted aquarium become a nice looking showpiece for your home.

-John N.
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