I've been studying this issue over the last 3 years as an artist and Marine-biologist-wannabe. In my research I have found so far that if the ceramic is fired cone 6 or higher the chemicals in Kaolin are inert. No leaching. I've tested for metals and other reactants as my low budget allows and found no traces of anything leaking or altering my tank chemistry. No shrimp, invert, or fishes dying.
Do not low fire the ceramic and expect it to be inert. In other words, do not bisque fire at cone 04 or 05 and put it in your tank. BUT if you refire them alone, do not place them with glazed pots sharing the kiln for firing, and fire them a second time at cone 6 (not to be confused with cone 06 - they're not the same) then the chemicals are locked in and as I said, "reef-safe" as far as I've been able to test and confirm.
With that said, I have yet to find longer term use that tested the water using ICP - MS (Inductively Coupled Plasma Mass spectrometry ) or ICP- OES (optical Emission spectrometry) to prove what chemicals may leach if they aren't rendered inert.
There is far more than Aluminum to worry about : lead, iron, mercury, Copper, PCBS, oxybenzone, dioxins, just to name a few.
Not all clay bodies are created equal, either.
white body - for example B-mix, is the safest I've found. You get into red clays, deep brown or black clays and they have more iron. Ironstone clay, terra cotta, Old Mine #4 ( this one is like 12% iron) are clays that typically have other metals too which all have cause for concern. Yet, I've seen countless uses of terra cotta pots as beds for clownies. For specific use alone in a breeding tank I suspect the iron, etc. is likely inert from the higher firing, or it’s just obviously not on issue for clownfish. But iron can boost algae growth explosevily, and then effect the ocean biome from there. I've read somewhere that iron may trap carbon dioxide, too, also altering ecology. Iron in your tank enough will definetly lower the pH below 8, and I've tested for and seen.
I would Love to know I anyone has tested their water using ICP tests? I know this may bring jokes about urinating or our ammonia producing fish - bring it on :)
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