Filter aid is made of the left over bone and shells of tiney dead sea critters called diatoms which is why its proper name is diatomousous earth or something like that. It is used as a organic bug killer also as its so rough it casues the bugs to wear holes in there skins and die.
For filtering it seems to gather all the small particles into big ones so it dosn't plug up the filters so fast. We used to feed it to baby calves to stop scours also years ago. I haven't heard of anybody useing it in the last couple of years for that.