I have read in various aquarium sources and forums that using standard unglazed clay flower pots is fine in fish tanks, so I've been using them as decoration, particularly caves. They're cheap, easy to break and sand to shape, easy to clean, and easy to replace. That said, I'm still a little wary.
As for porcelain - I don't have any expertise - but I think there could be a problem for any glazes used. I don't think it's the porcelain itself that could be a problem. For instance, a notice about health and safety for pottery makers (which is different - long term and concentrated exposure) shows a lot of health hazards - but this one caught my attention, even though it is primarily for pottery *makers* mostly - still -
Lead-glazed foodware can leach lead if not fired properly, or if the glaze composition is not correctly adjusted. For example, the addition of copper to lead frits renders a higher solubility of lead in the final fired ware. Acidic drinks and foods such as tomato juice, citric juices, sodas, tea, or coffee, can increase this hazard.
A glaze label marked "lead-safe" means that the finished ware, if fired properly, will not release lead into food or drink. The actual glaze is still hazardous to handle and fire and may contain lead. Adequate control over firing conditions is very difficult in the craft studio.
Other fluxes such as barium and lithium are also highly toxic by inhalation, but less so than lead.
Certain colorant compounds of particular metals are known or probable human carcinogens, including: arsenic, beryllium, cadmium, chromium (VI), nickel, and uranium.
Antimony, barium, cobalt, lead, lithium, manganese, and vanadium colorant compounds are highly toxic by inhalation.
Antimony, arsenic, chromium, vanadium, and nickel compounds are moderately toxic by skin contact.
Free silica occur in many of the clays, plant ash, flint, quartz feldspars, talcs, etc. used in glazes. See the discussion above for the hazards of silica and the disease silicosis. Weighing and mixing glazes can result in the inhalation of these toxic materials.
Soda ash, potassium carbonate, alkaline feldspars, and fluorspar used in glazes are skin irritants.
Spray application of glazes is very hazardous because of the potential inhalation of glaze mists.
Dipping, pouring, and brushing certain glazes may cause skin irritation and accidental ingestion due to careless personal hygiene habits.
Glazes containing solvents are both flammable and hazardous.
Some are more relevant than others for a fish tank - but I know nothing about this.
- mj
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