how about crabs or crayfish to make it look like a monster climbing on the hill like Godzilla? bottom dwellers in general could make it cool. An army of amano shrimp (strike that, cherry shrimp) scouring the hill side then BOOM, shrimp everywhere.
i am going to sleep happy with that image, happy and chuckling
what if you got some floss or some fishing line (maybe? i dunno), and then attached a bunch of it to a weight which is sunk into the volcano, bubbler would go off and it would wave and look pretty cool. I'm thinking the floss might also reflect some of the red light if you put a red light under the tank/under the volcano and then kept it debris free (so that the light would shine through)