For most people who didn't study in the US PPM might be a new way of expressing concentration of nutrients in the aquarium. I know it was for me when I started out reading about nutrients. Probably because the US doesn't use the decimal-based metric system, it lacks quick ways of converting. This has resulted in popular use of % ppt ( parts per thousand) ppm (parts per million) ppb ( parts per billion). And unfortunately PPM, is not a unit anymore than % is a unit. PPM is a dimensionless quantity.

#### Easy and short

In the aquarium hobby the meaning is:

1 ppm = 1 mg/L

## What are ppm

In the strictest and correct sense ppm is a *fraction* of two quantities of the same unit. It just says that 1 part for every 1 000 000 parts is our substance. Most will be familiar with percentages (%), which are 1 part for every 100 parts. And just like percentages it lacks any units.

Just like 300% PO_{4}-^{} has no meaning, in the strictest sense, 300ppm PO_{4}-^{} has no meaning. Thus lacking a unit it is very ambiguous. Is 300ppm by volume ? by mass? by number of atoms? what?

Sometimes you may see ppt, which most often means parts per thousand or 1 / 1000.

Here are graphical examples of the literal meaning of %, ppt and ppm.

1% - 1 part per hundred

1ppt - 1 part per thousand

1ppm - 1 part per million

## How is PPM used in aquarium keeping?

Short answer, it generally means mg/L. And as you may notice they are actually a unit for mass divided by a unit for volume... but we said it has to be the same unit.
So how did they arrive at '1ppm=1mg/L' ?

Given that most solutions are largely water (big assumption) and that the density of water is ~1 kg/L at most common temperatures (an approximation) you get the following:

1 mg/L = 1 mg / 1 L ≈ 1 mg / (1 L × 1 kg/L) = 1 mg / 1 kg = 1 mg / 1000000 mg = 1 ppm by mass

## Conclusion

And so, with an assumption and an approximation we ended up using ppm as a *de facto* unit of mass concentration in aquarium keeping. I would encourage you to write mg/L instead of ppm for clarity.