Hi Steve,
I think the material of the groundwork will look good if it is indeed fine grained enough and added subtle.
Perhaps you can make a mixture of it with white glue?
Using it will blend the tank in it's dio through the colour and structure of the material.
Perhaps a little drybrushed on some places to gain the effect of parts with dried up mud.
You can experiment a little on the bottom of the tank.
Succes!
Paul