Yet JBL says :
Ideal substrate
The substrate is an important part of the aquarium components because a lot of animals dig in the ground to search for food. The plants also need a safe hold and have to absorb the food in the ground through their roots. Digging fish absolutely need a substrate which is not sharp-edged. Undergravel heating can only be recommended for medium-fine to coarse substrates because water circulation cannot really build up in very fine substrates.
Use:
Rinse the substrate with tap water to remove any abrasion caused in transit. Put substrate on the nutrient substrate (JBL AquaBasis) to promote the plant growth (for vigorous growth about 6-8 cm).
Calculation formula
The right amount of substrate: length (cm) x width (cm) : 140 = kg substrate with 5 cm thickness.