I have used Tufa for 25 years in both freshwater and saltwater environments. Its excellent. The porousity is great for bacteria growth, it grows coraline well and is a much lighter rock for creating rockwork. It is 97% calcium carbonate with other trace elements(no metals) and is mainly formed from underground fast running freshwater streams that are very calcium rich. It starts as a foam underground and solidifies on its way to the surface as it is pushed up by more foam formation under it. It does not break down in water. It can be broken easily, but does not naturally decay. I imagine if your PH got down around 7 it would start to break down in the same manner as calcium reactor material, since is has such a high calcium content.
$3 is a little high, but not crazy high. When I used to sell some I would get $1.50 a pound and $2 a pound for the cool looking pieces.
Biggest thing is to make sure it is power washed WELL. The fact that it is so porous means there is allot of dirt generally packed in it. I would soak it for a week, power wash it, soak it for another week, powerwash it again, let it dry out, then powerwash it one more time before I would sell any to make sure it was clean for sure.
