How to read fertilizer labels, and what is ppm ?

We usually give liquid concentrations as percentages. So if a lab technician dissolves 1 gram of "sugar" in 10 grams of water, they will get a solution that is "10% sugar". Makes sense, doesn't it? Another way of expressing this concentration is as 1 part per 10.

However, because the concentration of fertiliser in our tank water is so much lower, we will instead give the concentrations in ppm (parts per million), which for our purposes is the same as mg/L (milligrams per litre). Generally, aquarists would use ppm as the basis for measuring and describing the amount of fertiliser to use/dose.

1 mg/L is the same as 1ppm

Having 1 ppm of potassium in my planted tank means that if I divide the total water mass of my planted tank by 1 million, 1 part of that weight is potassium.

The above bottle states in the box text that dosing 5ml of Flourish Potassium into a 125L/30gallon aquarium will increase the potassium level in the tank by 2ppm.

In another post, we described 2 popular approaches to dosing, EI and ADA.

How much Flourish potassium do I need to dose to reach 20ppm, the approximate weekly dosage rate used in both the EI and ADA dosing approaches? Extrapolating proportionally from the ratio given in bold below, it is 50ml (10 times the 2ppm in each 5ml). So if you were to copy either the EI or ADA dosing approach for potassium, you would dose 50ml per week using Seachem Potassium. You may wish to split the weekly dose to even out the dosage, so if you are dosing daily, it will be 50/7 ~ approximately 7ml daily.

Note that the figures are rounded for simplicity.

Figures in bold are the amounts shown on the label. The preceding and following lines are proportional extrapolations. The right hand column gives the dosage required to achieve 20ppm (10 times the 2ppm that is given in each 5ml dose). By similar extension, if we wanted to get, say, 10ppm of potassium in the same 125L tank (instead of 20ppm), we would dose 25ml (5x 2ppm) of Flourish Potassium.

​The next question is how much to dose for each element?

​For this we can look at some common dosing approaches used by aquarists. EI is a good example of a high level of liquid fertiliser dosing, designed to provide large amounts of nutrients to achieve a high rate of growth, whereas ADA dosing is much leaner and relies heavily on the substrate to provide nutrients. Go here for details on EI and ADA approach.