Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Concrete: The more water changes the better, but allow a month.
Black expanding foam, sold for ponds, water falls and similar: Great material! Does not expand as much as the largest expanding construction material, but it does expand. It is much stickier than the construction material, too. Once it gets onto your skin you are marked for several weeks! I do not find it irritating to the skin, but if you are sensitive to things, wear gloves. Completely fish safe once is has cured. Sticky enough to act as a glue to hold the rock in some other firmer, shaped base. I would not use it by itself. Maybe it would make a skin over the concrete, and the color sure is better than concrete!
Epoxy (2 part) paint over concrete. Look for the potable water type. Very expensive, but it will seal in the minerals from the concrete.
Other types of concrete sealers: I would not trust them under water. They are for above ground use for sealing patios and driveways. In this use they need to be re-applied every few years. They wear off. I have no idea if they are fish safe, or if they would even seal the concrete enough to prevent leaching of the minerals.
Styrofoam: Is OK in the tank, if you can make it work for the rock. The expanding foam might make a nice cover/sealer for this. Styrofoam is used for aquarium backgrounds, usually with colored concrete for texture and color. And weight.