In order to
calculate how many liters you require you need to figure out the cubic inches
of the aquarium. So for example a
standard 50g long would be calculated as follows.
Tank width= 12 inches
Desired substrate depth= 3 inches
48x12x3= 1728 cubic inches.
To convert to liters, you can go here
In this case
it’s about 28 liters or 3 full 9 liter bags and a bit.