Glass is really just sand heated to very high temperature. I've been told that glass does not leach silica into tanks (ie tanks are made of glass) Ceramic glazes are essentially the same thing with pigments to make the colors. They are fired usually above 2000 degrees. The clay is also fired at these temperatures. There are many different clays and many different glazes. I tend to use only "non toxic" ones. But what is nontoxic to humans may not necessarily be nontoxic to fish. I guess I should email the glaze and clay companies to see if they have done chemicle testing for "leaching" and see if perhaps they know the answer. otherwise I guess the thing to do is set up a hospital tank and stick a piece of my pottery in it for months and find a way to test the water. Maybe the local township water dept can test it for everything. I'm wondering about all of this because I hate to be using live rock and contributing to the dismanteling of coral reefs. Right now I have only two large pieces of live rock in a 200 gallon tank.. The rest is dead coral I have had for years and "reef replicas" algae grows anyway on my dead corals and my reef replicas. I have 0 nitrites and 0 nitrates and I have copepods as well. If we could make ceramic (or even plastic) replicas impregnated with millions of tiny holes, wouldn't the bacteria and algaes just grow all over it anyway? I will email Spectrum and Duncan glaze manufacturers today and see if they know the answer.