Aquarium safe pottery/china?
 

I've got some Cabomba that does not want to stay rooted in my tank, as it
the stems keep rotting away. So it's temporarily floating around and growing
roots from every orifice. I have read that Cabomba roots better in soil,
rather than gravel/fluorite. I'd like to set up some pots with a layer of
fluorite, a layer of soil and a top layer of ???? to keep the soil down
without damaging the Cabomba stems. I bought some black pasta bowls (made in
China) for the task, but I'm concerned that they might leach nasty things
into my tank. They are rated as dishwasher safe, but that doesn't mean they
will survive full immersion or not leach from the non glazed bottom.

Any thoughts?