You have to watch using clay post that have had plants in them. Sometimes there have been fertilizers or pesticides put on the plants and clay pots will absorb all that.
Dano if your not set on clay pots check these out. click on the link then scroll down to crafts. Carol(not me) makes all kinds of breeding caves and stuff like that. They are really well made.
while your there check out their fish, as they are breeders.