Q. I am writing to you with a few questions about aquascaping. I can obtain clay field tiles to use in my African cichlid tanks. These tiles are new and have never been used in farm field drainage. I want to know if they can be used safely in my aquarium. I also have plenty of used flowerpots. What should I do to prepare them for use in my aquarium?

My last question has to do with geodes. I would like to use them to decorate a show tank to display my fish. I have been told that minerals in the geodes will leach into the water and kill my fish. Is there any type of geode that I can safely use in an aquarium? If so, what should I do to treat it before placing it in the tank?

A. Unglazed clay tiles — particularly unused tiles — are perfectly safe for use in an aquarium. Simply rinse thoroughly to remove any dust that may have accumulated on their surfaces and add them to the tank.

The same protocol applies to virgin clay flowerpots. In the case of used flowerpots, scrub them thoroughly with a stiff brush to remove any soil or salt deposits that may have accumulated on their surfaces, then soak them for a few days to leach out any fertilizer residues that may remain. Three or four water changes should effectively remove any residual nitrates.

The geode question is a bit trickier. The crystals within the geode are some sort of silicate mineral, and as such are effectively inert. Any problems are likely to come from the surrounding matrix rock.

One can determine if a rock is likely to increase the pH and hardness of an aquarium's water by placing a few drops of plumber's muriatic acid (HCl) on its surface. If the acid effervesces as soon as it strikes the mineral's surface, the basic composition of the matrix is alkaline. Such rocks will raise both the pH and hardness of the water in any tank into which they are placed.

The problem is that there are no comparably simple tests for toxic metals. Under these circumstances, the only way to test the effects of the geode's matrix on fish would be to set up a bioassay by rinsing the specimen thoroughly, placing it in a small tank containing one or two "dispensable" fish and monitoring their health for seven to 10 days. If all the fish are still alive and in good health at the end of that time, the geode can be safely added to your display tank.