The Estimative Index: nutrients aplenty

The secret to a beautiful, healthy planted aquarium? A good fertilization schedule combined with sufficient carbon (CO2), balanced lighting and adequate water circulation. Quite a task, but fortunately the Estimative Index makes the fertilization part easy.

The idea of the Estimative Index is actually quite simple: you add nutrients to your aquarium until you get a surplus. This way you are sure that your aquarium plants will never have a nutrient deficiency and can grow optimally. “Nutrients aplenty” so to speak, and believe me: your aquarium plants love it.

What is the Estimative Index and who created it?

The Estimative Index, or EI for short, is a dosing method for aquarium plants built on a single principle: make sure a nutrient deficiency can never be the cause of a problem. Every week you give your plants a slight surplus of nutrients, so they always have everything they need. At the end of the week you do a substantial water change, and that resets the balance for the next dose. This takes one big unknown out of your tank: if something still goes wrong, you know it is not the fertilization.

The method was developed by Tom Barr, an American biologist specializing in aquatic plants. His core insight: most algae problems do not come from too many nutrients, but from weak plant growth caused by a lack of light, CO2 or nutrients [1]. Algae themselves are rarely limited by CO2, whereas aquatic plants often are. Let your plants weaken and you give algae room. Strong, well fed plants keep algae under control instead [2]. That is exactly why the EI has you dose a small surplus on purpose: that surplus does not cause algae, which Tom Barr demonstrated in his own tanks [3].

The name says it already: the Estimative Index is an estimation method. Because you add a generous, fixed amount of fertilizer, you no longer need to measure with test kits. And that is just as well, because most commercial testers are pretty unreliable. You have probably noticed it yourself: brand A’s test kit gives a different result than brand B’s, or you have to guess at the colour.

So measuring has become guessing. The danger of this is that you easily draw the wrong conclusions and end up using the wrong fertilization schedule, which ultimately harms your plants. Moreover, with testers you only take action once it is already too late: the nutrient deficiency is there and the damage to the plants has been done.

My All-in-One is built exactly on the EI ratios, so you can follow the method without weighing, mixing or calculating. Would you rather prepare your own solutions? That is possible with the Estimative Index Kit.

How does the Estimative Index work

How do you apply EI to your own aquarium? It is quite simple: it is enough to add fertilizer daily or 2-3 times a week. In aquariums with low light and/or low plant mass, you can even proceed on a weekly basis.

The fertilizers you add are divided into two major groups. On the one hand, you have the “micro nutrients” (trace elements) such as iron, magnesium, manganese, … On the other hand, you have the “macro nutrients” such as nitrate, phosphate and potassium. The weekly minimum and maximum guideline values you can add to your tank are as follows:

• Nitrates: 10 to 30ppm
• Phosphates: 1 to 3ppm
• Potassium: 10 to 30ppm
• Iron & trace elements: 0,5 to 1ppm

For the trace elements we base ourselves on the amount of iron contained in the fertilizer, but you can always add extra iron with an aquarium iron fertilizer. Exactly how many milliliters you need to add to your tank to get to the above ppm values is listed on the back of the fertilizer bottle, at least in the case of the PlantedBox fertilizers.

If you want to make your own DIY aquarium fertilizers you can use a calculator or the provided recipe of the PlantedBox DIY kit. So let’s say you want to achieve a value of 10ppm per 10ml of KNO3 on 100 liters of aquarium water, then the calculator will tell you how many grams of dry salts you need to add to your bottle. If you are using PlantedBox fertilizers, the quick start guide in the DIY fertilizer kit will tell you how many spoons you need to add to your bottle for the correct dosage. It’s super easy.

On day 7, do a heavy water change, preferably about 50% of the water. The water change serves to limit the amount of nutrients in your aquarium. Suppose you dose 20ppm of nitrate and you do a 50% weekly change, you can never get more than 40ppm of nitrate. However, that maximum limit applies to situations where your plants absorb no nutrients at all. Usually, in reality, the maximum limit is much lower because your plants are always going to absorb some fertilizer as nutrition, especially if plant growth is healthy.

A second function of water changes is to get rid of the many waste products your plants produce. As they start growing stronger and faster with the Estimative Index, they also start creating more waste. If this waste is left too long in your aquarium, it will rot, which can cause algae. So by doing regular water changes you reduce the risk of algae. In addition, you introduce a lot of new minerals and CO2 into your tank with the tap water, something your plants will certainly appreciate.

Do you have a very large aquarium and 50% refreshing is not so easy? No problem, you can still use the Estimative Index while doing fewer changes by adding less nutrients. An example: suppose you add 30ppm nitrate and do a 50% refresh and you want to switch to a water change rate of about 25%, then it is sufficient to add 15ppm nitrate.

Does the Estimative Index cause algae?

Many aquarists fear that with the high nutrient values of the Estimative Index, they will have algae. However, that fear is unfounded. Just look at the pictures in this article, these are the tanks with values even higher than those described above, and yet: no algae to be seen.

Why is that? Because the fertilizers we use for Estimative Index dosing is an “inorganic” fertilizer. These products are already in their final stages, there is no rotting process taking place and thus no algae-causing substances. The statement that nitrate & phosphate cause algae in aquariums is actually a holdover from the days when we added those fertilizers via fish food. That fish food would then rot and be converted by the bacteria to nitrates and phosphates. During the rotting process, however, substances were created that caused algae (e.g. ammonium). Unfortunately, the link was wrongly made between nitrate/phosphate and algae and not ammonium and algae. Fortunately, with today’s fertilizers, this is no longer an issue.

Another cause of this erroneous statement has to do with the way growth and demand are linked. Tom Barr puts it plainly: as plant biomass and growth rate increase, the demand for CO2, nitrate and potassium increases along with them [4]. If you add extra nutrients while your CO2 is already only just sufficient, you speed up growth and you widen the CO2 gap. The result: you see more algae and you think it’s because of the extra fertilizer. However, this is not correct since the direct cause of your algae is the CO2 deficiency, the fertilizers only highlighted this problem. In that case, you need to work on your CO2 first and certainly not dose less nitrate and phosphate, because by doing so you risk causing another deficiency.

If you really want a healthy plant tank, then your main focus should be on the health of your plants and not on limiting algae. An aquarium in which the plants live in ideal conditions with sufficient CO2 and plenty of nutrients will have virtually no algae. A tank where you take a preventive approach and limit fertilization out of fear of algae runs a much greater risk of nutrient deficiency and consequently algae. And this can become a dangerous vicious circle after a while.

In short: provide sufficient nutrition so that your plants grow well and the algae will stay away by themselves.

Is the Estimative Index safe for fish?

Another comment you often hear is that the Estimative Index is bad for the health of your fish and shrimp. Let’s look at the numbers.

Tom Barr exposed fish to 120 ppm of nitrate for three days, derived entirely from KNO3. The result: the health of twenty South American fish species was not affected [5]. With the Estimative Index you aim for 10 to 30 ppm of nitrate. So you stay well below the level at which he saw no effect on the fish at all.

One honest caveat, and it matters: in that same experiment, Amano shrimp (Caridina japonica) turned out to be considerably more sensitive, with 50% mortality at those same 120 ppm over three days [5]. Your shrimp tolerate less than your fish do. At the EI dose of 10 to 30 ppm you keep a comfortable margin, but if you run a shrimp tank, keep that difference in sensitivity in mind and do not let your nitrate creep up.

It is more difficult for the other substances because the academic research on them does not exist or is difficult to compare within the context of aquaria. For the toxicity of phosphate, for example, studies do exist, but they were conducted on European lakes, a biotope totally different from our heavily planted aquariums.

So for much of our fertilizers, we are assigned to practice, which shows us that the values we use are fine for our fish. In fact, sensitive fish such as Sturisomas and Discus were found to be able to breed in Estimative Index tanks. The pictures in this article are proof of that.

Some remarks

The Estimative Index is an effective tool, but not a panacea that will turn your tank into a lush jungle.

Many other factors play an important role in the health of your plants like aquarium lighting, filtration, water circulation and CO2. The latter in particular is a sticking point with aquarists. If you do not have an optimal CO2 level in your tank, the Estimative Index will not solve this and your plants will still grow poorly. The only solution then is to increase your CO2 supply and continue dosing according to the Estimative Index.

In such situations, the Estimative Index is ideal for determining the cause of your problems. Suppose your plants are growing poorly and you suspect a nutrient deficiency, you can use the EI to exclude nitrates, phosphates, potassium & trace elements. What then remains are CO2, lighting and water circulation/filtration.

Sources

[1] Barr Report Forum. Algae: liquid versus injected CO2 preference. barrreport.com
[2] Walstad, D. “Ecology of the Planted Aquarium” (1999), Chapter X “Algae Control”, p. 157, 159.
[3] Barr, T. “Phosphorus’ Role in Aquatic Macrophyte Horticulture.” Barr Report, Vol. 2, Issue 1, January 2006.
[4] Barr, T. “Growth Relations of Aquatic Macrophytes, Volume 1.” Barr Report, Vol. 2, Issue 10, October 2006.
[5] Barr, T. “Nitrogen Cycling in Planted Aquariums.” Barr Report, Vol. 1, Issue 6, June 2005.

Last updated on 13 July 2026