Ok now its a little bit difficult, my english is not good enough, but i'll try:
My experiance with freshwatersand ist not that bad, therefore I can follow Colin. The silicate??? (SI) in the freshwatersand let to grow some Algae (Kiselalgen), but if they are to much use an silicat-adsorber.
Fine Sand is better than rough sand in the tank, because the fine sand has a much bigger surface for the bacteria.
The sand should not have sharp edges, that can hurt the skin of the okto.
I hope your able to encrypt my text.
Chris