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How to read fertilizer labels, and what is ppm ?

August 17, 2020 2 min read

How to read fertilizer labels, and what is ppm ?

We generally quote liquid concentrations in percentage terms. So if a laboratory technician dissolves 1 gram of "Sugar" in 10 grams of water, he will get a solution that is "10% Sugar". Makes sense, doesn't it? Another way to express this concentration is as 1 parts per 10.

However, because the concentration of fertilizers is so much smaller in our tank water, we quote the concentrations in ppm (parts per million) instead, 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 up my planted tank's total water mass by 1 million, 1 part of that weight will be potassium.

In the bottle above, is stated in the boxed text: dosing 5 ml of Flourish potassium into an aquarium of 125L/30gallons will raise potassium levels in the tank by 2ppm.

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

NUTRIENT (weekly dosage), ppm EI  ADA
Potassium (K)
Nitrates (NO3) 15~20 1.5~6
Phosphates (PO4) 2~6 1.4~4
Magnesium (Mg) 5~10 undefined
Iron (Fe)
0.5~1 0.03~0.06

How much Flourish potassium must I dose to reach 20ppm, approximately the weekly dosage rate used in both EI and ADA dosing approaches? By proportionately extrapolating from the stated ratio in bold below, it is 50ml (10 times the 2ppm that is present in every 5ml). So if you are copying either the EI or ADA dosing approach, for potassium, you would dose 50ml per week using Seachem Potassium. You might want to divide up the weekly dosage to even out the dosing; so if you do daily dosing, it will be dosing 50/7 ~ approximately 7ml daily.

Note that the figures are rounded for simplicity.

Tank Volume (L) Tank Volume (US Gal) Amount to dose to reach 2ppm  Amount to dose to reach 20ppm
~40 ~10 ~2ml ~20ml
~75 ~20 ~3ml ~30ml
~125 ~30 5ml 50ml
~150 ~40 ~7ml ~70ml
~200 ~50 ~8ml ~80ml
~230 ~60 ~10ml ~100ml

The figures in bold are the amounts indicated on the label. The preceding and subsequent rows are proportionate extrapolations. The rightmost column indicates the dosage amount to reach 20ppm (10 times the 2ppm that comes with each 5ml dosage). By similar extension, if we want to to get say 10ppm of potassium in the same tank of 125L (instead of 20ppm),  we would dose 25 ml (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 that aquarists use. EI is a good representative of a high level of liquid fertilizer dosing, designed to provide large amounts of nutrients to achieve high growth speed, while the ADA dosing regime is much leaner, and depends heavily on the substrate to provide nutrients. Go here for details on EIand ADAapproach.