Hi,
It is sheets of polystyrene that are stuck together.
So first I workout what size it must be to fit in the tank,
Then start off with stacking the the pieces on top of each other I used off cuts to mock build, to find some thing that looks nice.
Once happy with the shape and every thing then it a matter of gluing the pieces that fits together, and then let it dry with a bit of weight.
I use wood glue to stick it together.
Once it has dried then I go and cut and sand to give it shape!
And then comes the fun part, is painting it with Tile grout, I found the Black tile grout is the strongest.
Mix your grout mix, that it is runny but still thick enough to paint with a paint brush, the thicker your mix the harder it dries, and the less coats you have to paint onto it.
Often people use different colors of grout or paint just to show where they stopped and how many coats the have put on? but mainly to show you where you have and have not painted!
Then you wait for it to dry, and then you can start painting, with a water based paint, can give it a couple of coats say about 4 just to seal it.
I will try do a How to once I have a couple of hours and another tank to build a design for!
Hope that helps?