What material do i use for DIY aquarium decor?

[Poklei]

do i use clay?

 

[mrwinkle]

yes if you have it fired in a klin

 

[the_deeb]

There are lots of options. People often use styrofoam/polyurethane foam coated with some sort of concrete, painted and sealed. Another option is sculptable epoxy (eg. Zoopoxy from Polygem) which allows very realistic work but is expensive.