I used colored, sanded grout over an expanded polystyrene foam model. I first made the model, then distressed it, then coated with up to 3 coats of grout in varying degrees of runniness(slump). Before I put the grout on I cut the model into pieces that I could easily put in and out of the tank. After grouting it,I siliconed it to the tank and put a last coat of grout on the silicone seam. Now I am waiting for it to cure as has been decribed on this site by others.
Have a nice day!