EI aims to dose all nutrients in excess through heavily dousing the water column with nutrients. Levels are normalised at the end of each week with large (50%) water changes. The aim is to provide plants with non-limiting amounts of nutrients.
The EI philosophy in simplified form is as follows: Fundamentally, it is difficult to gauge with precision the exact uptake rates in a tank for any particular nutrient. This is especially so when the tank has different types of plants and other growth parameters involved (light/CO2). However, dosing everything "in excess" removes any possible nutrient deficiency from the analysis (we assume plants are getting all the nutrients they require through the plentiful amount in the water column) and allows the aquarist to focus on tweaking the other growth variables: light, CO2 levels (this is usually the hardest thing to get right), and other water parameters.
But what about algae? EI theory is based on the idea that if plant growth is taken cared of, plant mass will out-compete algae. It says that algae presence is not determined by whether nutrients are available or not, but rather whether or not there is sufficient plant mass to compete algae (not only in terms of nutrients, but space, light). Healthy plants are the best defence against algae. This is covered more in the portion about diagnosing algae issues.
A weekly cumulative dose of nutrients according to EI would be around:
NO3 - up to 20 - 30ppm
PO4 - up to 5 - 7ppm
K - up to 20 - 30ppm
Fe/traces - up to 0.5 - 1ppm
Divide the above by number of days dosing is done to get a daily dosing rate. Some people use higher/lower levels compared to the usual recommendations, and many tanks are flexible enough that things work in a large range.
Tom Barr's tanks (such as the one above) are often used as the marketing face for EI. However, his success is more closely tied to good maintenance, quality of lighting and upkeep than on the actual nutrient ratios used, but people tend to get caught up with the nutrient angle. Tom Barr campaigns strongly for the idea that high levels of nutrients in the water column do not cause algae issues - if plant mass was dominant & healthy in the tank.
The rich red colors of many 'Dutch style' tanks are largely the impact of lighting - and not because of particularly heavy nutrient dosing into the water column, a concept that many beginners are mistaken on. Indeed, certain plant species such as Rotala rotundifolia & Ludwigia arcuata/brevipes are significantly redder when grown under a lean dosing regime rather than EI.
Tanks that are suitable for EI dosing approach:
Tanks that will probably do better on a leaner dosing approach:
Here are the links for further reading.
1. Detailed care guide to aquarium plants
2. How to grow red aquarium plants