Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Use the raw, unfired product, as it is dug out, and mix it with the lower layer, not the cap. Probably do not need much, either. Look into Mineralized Top Soil for the actual recipe.
This question has come up before, and the consensus was do not bother using broken pots. Once the clay is fired it may not have the same (useful) properties.