[nickman]

you can always screw a chunk of slate to the bottom with stainless steel screws like the stuff you get in fish stores. you have to get a masonry bit to drill a hole in the slate however.
its not a big deal but sometimes even a rock wont be enough at first.
this is why i collect my wood from under water! haha
or use the malaysian/african stuff that sinks right away, im not the most patient person.

-nick

 

[nickman]

there should be no problems with a stainless steel screw. most impellar shafts are stainless and those see alot more water circulation than something sitting under the substrate. rust is just oxidized iron. we add iron to our tanks all the time. other metals may present a prob esp with inverts.

take a look at a mounted piece of driftwood the next time you go to the store. there's a screw in the bottom. you can also reposition the wood on the slate if you change your mind. good luck with that using glue or epoxy.

-nick