I use terra cotta in all my tanks (except for the big one). I've never noticed any abnormal test results with the water. I'm sure plastic would be ok too, after all, most tank decorations are made of plastic. I just think the terra cotta is a nicer color/texture.JWhipple said:I'm thinking about putting some flower pots in with my Viejas ... Which is preferred - clay, or plastic? Does the clay do anything to the water chemistry?