From what I have researched, what has undergone testing as to it's effectiveness, the 8x24 is what was used. It was shown to be most effective when used by itself as the active absorption media. A very fine mesh to keep any particles from escaping, then washed fine sand, then the Bone Char and finally another layer of sand (larger particles) on the inlet side followed by a a very fine mesh to catch the sand and if you are back-washing to try to get more out of it before replacing the char, it might make the backwash more effective. That being said, no other sizes of particles were in the testing data that I read. A larger size of particle might be harder to contain and keep from getting into your water (so you might want to let the water run through the filter for awhile just to wash out the smallest particles that may end up contaminating your fluoride reduced water, and a larger particle size might be more effective or less effective at reducing fluoride, etc. One important thing is that the water spend some time around the particles of char before going through the filter, giving the char time to absorb contaminants. If the water flow is somewhat slower going through to be cleaned than the backwash water, (then you hopefully have kept the medium size particles in your tank), and perhaps those are the ones you want to keep
. Also, there is a manufacturer of char that I found the name of by reading the reviews of a bone char sold on Amazon who has a sample of that size they will sell for less than $10. (So you could get an idea of how fine the 8x24 is). But this is an old post and I'm sure you are well on the way to filtering!
