I know beyond a doubt that psyllium has helped remove sand from the gut. I used to have an old Sheltand pony who would sand colic regularly. The first sign of a problem was soft, runny, heavy manure, and if I gave psyllium right away I could avoid a colic. If I found the pony already not eating his hay I would make him a mash with a cup or more of psyllium mixed with bran or sweet feed (to make it taste good, bran does nothing for sand) and lots of water. If he was too sick to be tempted to eat that, I would mix the psyllium with water and syringe it into his mouth. Within 2-5 hours (as I remember) he would poop and it would be loaded with sand. The ground is pure sand where I live near Ridgecrest, CA, and the vet recommends psyllium. She said this old pony probably had a damaged gut from sand, with stretched out pockets that would collect the sand easily. I use generic metamucil because it's cheaper and handy to buy. This happened so many times I'm convinced it worked.
I'm not so sure if it has an effect on intestinal stones, but after I bought my POA , Rosie, I gave her metamucil and she passed many 1-2 inch stones. She was on straight alfalfa before I got her, and after switching to grass hay I haven't seen another stone.
I used to have the horses on the "once a month for 5 days" regime, but haven't done it for a long time. After reading these posts I think I'll do it again. After metamucil is mixed with water it gets kind of sticky and gelatinous, and I guess the sand gets suspended in it.
Linda