den. kinganium media problem - plz help

[Erce]

i have a nice small den. king who i have been slowly killing for long time now - it had nice roots, then i gave it far to much water, killing all its roots.
Then i grow some new ones, and i am now so so scared to kill them to - having a big problem to know when to water - my media seem to hold far to much water for this guy!

Then i wonder - i have 8L of vulcan rook - it is small stones and gray - it is sold here in denmark as a growing media for people with allergy.
can this be use as only media for my den?