I don't know the answer directly, but I can extrapolate from what I know of cement & fish pond.
Main problem with cement is that it is very alkaline, and will change the water pH for a long time to come. From reading about koi ponds, it is recommended to leave a new cement pond empty of fish for at least a year so the cement will be totally cured. That would involve weathering outside with water in the pond, etc. I supposed if you make a cement ornament, you would need to do the same thing to make sure that the cement is totally inert before adding it to the tank.
The addition of peat to tufa mix is to lighten the cement. The peat will disintegrate, leaving small holes. During the rotting processd, it will prob. change the water chemistry. <We do use peat to lower pH, so I expect the same effect.> However, if you weather the cement object completely, the peat would be long gone by then.
Anyway, it seems that you need a long time to let cement cure to the point of being inert. If you were to use it for fish, you would definitely need to test the object to make sure it is totally cured. <Soak in water for a few weeks & make sure it is inert.> A better alternative would be to use food grade molding plastics/silicone (plastics supply or modeling hobby place). At least you won't have to wait years before using your creation!
