“Estimative Index” aquarium plant fertiliser method and strategy

The ‘estimative index’ (EI) is a fertilizer concept originated by Tom Barr where the general idea is to never have plant nutrients be limiting in the water column.  Plants will grow and generally outcompete algae (so the theory goes).  In practice, while I’m sure the outcompeting part is a factor, the algaecidal properties of “liquid carbon” are doubtless also a big help.  In any event there are a couple of attractive features to the concept:

  1. “excess” fertilizer does not promote growth of algae over other plants, so you don’t have to worry about for example having your nitrates or phosphates be super-low as an attempted algae control measure
  2. it’s good enough to estimate how much fertilizer you’re going to need, so you can do some upfront calculations and then there’s no need for a lot of expensive/inaccurate water test kits.  Sort of a set-it and forget-it approach.

Fertilizers are divided into two types:  ‘macros’ which includes the big ones like nitrogen, phosphorus and potassium (the ‘NPK’ reading in fertilizers) and the ‘micros’ including iron, copper (not too much), cobalt, manganese etc.  Macros are dosed on days 1,3,5 each week and micros are dosed on days 2,4,6 each week.  Day 7 you rest (heh) and then on day 1 of the next week, you do a 50% water change, and then start the cycle again (dosing the macros right after the water change).  If you’re doing liquid carbon, you can dose that every day.  You don’t dose the macros and micros on the same day because (so the story goes) the iron in the micros will form insoluble precipitates with components of the macros.  I’m not sure how true that is, but alternating the dosing days is simple enough.  The 50% weekly water change is important to reset the amount of fertilizers so you don’t get a weekly buildup.

Your most cost effective and customisable way is to mix your own macro fertilizer solutions and micro fertilizer solutions from “dry salts” which is to say, from actual pure chemical components.  You can get these from many places, but I like AquariumPlantFood which has affordable and convenient options.

Fireplace Aquarium macro fertiliser concentrations

Target fertiliser concentrationsMacro mix componentsActual weekly ppm contributed
I make up 500 ml solutions each of macro and micro and on the dosing days I add either 8 ml of the macro solution or 8 ml of the micro solution to the estimated 40 L of water in the aquarium. My current weekly dosing target (so split over the 3 doses per week) is:To get to those concentrations I make a stock solution of macros by dissolving the salts with water up to a total volume of 500 ml of distilled or deionised water*:

Note: the salts will dissolve more easily if you heat the water first
When you math it all up the actual concentrations are a pretty good match to the targets. They’re not an exact match but that’s ok because we have an ‘Estimative’ philosophy here, and anyway the contribution from components in tap water used in the weekly water changes is also unknown.
  • 25 ppm potassium

  • 15 ppm nitrate

  • 7 ppm phosphate

  • 5 ppm magnesium
  • 3.5 tsp KNO3 (21g)9.75 K, 15.45 NO3
    1.25 tsp KH2PO4 (8.25g)2.84 K, 6.91 PO4
    8.25 tsp MgSO4-7H2O (42.075g)4.98 Mg, 19.68 SO4
    4.5 tsp K2SO4 (22.95g)12.36 K, 15.19 SO4
    Overall, this works out to 24.9 ppm K, 6.9 ppm PO4, 15.5 ppm NO3, 5.0 ppm Mg, plus whatever is in the tap water used for water changes - don't worry about that. The sulphate won't hurt anything so don't worry about that either.

    I don’t do any calculations** for the trace elements in the micros; I just use a purchased pre-mixed dry salt pouch.

    There’s no real detrimental upper limit to how much sulphate you have in the water, so if you want to boost up one of the components, adding the sulphate version is good.  I like more potassium for the plants which is why I add the K2SO4 (this is listed as ‘optional’ rather than part of the core EI method as such).  I’m pretty convinced that having higher phosphate than recommended by the core EI method helps to suppress green spot algae, so I boost that up as well.

    The fish, shrimp and snails seem completely unperturbed by the whole process.  Since the Shrimphaus is roughly 20 litres vs. the 40 litres of the Fireplace Aquarium, the Shrimphaus gets daily half-doses.

    Preventing mold and stabilising chelated iron

    Both macro and micro mixes will support mold growth over time.  Also, eventually the chelated iron in the micro mix can precipitate out as a brown residue.  To prevent both of these from happening, each 500 ml mix can be supplemented with 1/8 tsp potassium sorbate and 2 ml vinegar.  The overall goal is 0.3 g potassium sorbate and 0.1 acetic acid, so the previous are based on potassium sorbate granules with a density of 0.46 g/ml and vinegar as a 5% solution of acetic acid.  The sorbate is an anti-fungal preservative often used in food, and the acetic acid keeps the pH low which stabilises the capacity of EDTA to hold iron in chelated form.  There is some thought that the lower pH also helps the sorbate function as a preservative so it is simple enough to add both to each of the master EI mixes.

    Online Estimative Index resources and calculators


    *Macro mix made up with tap water

    I have been using deionised/distilled water to make up the macro mix stock solution, but others report success with tap water.  If using tap water in an area with a hard water supply, it can be helpful to use pre-boiled water to ensure any calcium or magnesium bicarbonates have an opportunity to precipitate out as carbonates.  Otherwise the free Ca+2 or Mg+2 from the tap water can precipitate as insoluble phosphates in the macro mix bottle.

    **Aquarium micro nutrient concentrations

    The micro nutrients come in at around 1/100th the level of the macro nutrients.

    Mathing up the Chelated Trace components (for the sake of completeness).  1 tsp (6g) mix per 500 ml stock solution, adding 8 ml stock x3 per week.  I now also add 5 ml vinegar to the 500 ml micro nutrient solution to keep the pH down which stabilises the chelated iron (see for example discussion on UKAPS).

    Dry powder mix

      • Fe 8.2% (EDTA Chelated)
      • Mn 1.82% (EDTA Chelated)
      • Zn 1.16% (EDTA Chelated)
      • B 1.05%
      • Cu 0.23% (EDTA Chelated)
      • Mo 0.15%

    Weekly in tank ppm

    • 0.590
    • 0.131
    • 0.084
    • 0.076
    • 0.017
    • 0.011

    The plants look good with no obvious signs of micronutrient deficiences so I’ll just keep on keeping on with these.

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