What grain size should the gravel be?

A common gravel grain size is between 2 and 6 mm, which covers most aquarium biotopes. Gravel coarser than 12 mm is rarely used.

In nature, however, gravel is not always the same size, so a mixture can also be used. This looks more natural than if the gravel always has the same diameter.

When buying aquarium gravel, make sure that it is not sharp-edged. Corydoras and other fish with barbels like to dig in the substrate and can otherwise injure themselves.

You can also create a corner with aquarium sand. Sand in large layers is pressed tightly together and can lead to oxygen-free zones and the roots die off. This can lead to putrefaction centres with the formation of H2S, which can be fatal for the fish when released.