Is modelling clay reef safe?

[bigyankfan]

I was thinking of making supports to hold up pieces of shelf rock by cutting PVC to length and then wrapping modelling clay (or something similar) around the outside to give it a rock-like appearance. Any ideas?

 

[qy7400]

Great Stuff foam. http://greatstuff.dow.com/greatstuff/diy/products/pond.htm