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PPM is parts per million. This gives a ratio of – 1 to 1,000,000
Mg/l is milligrams per litre.
It is often said that these units of measurements are the same, and in certain situations they are.
Room temperature, pure water, at sea level pressure weighs exactly 1KG per litre. 1mg is 1 millionth of 1kg, so in this situation mg/l and PPM are identical.
When water temperature, the purity of the water, and air pressure change, the two units move away from each other.
However, in an aquarium or pond situation these variables make such a small amount of difference the two can be interchanged.