How much psyllium fiber is safe for horses?

This is not a good idea. This can cause digestive distress, just what you are, in theory, trying to prevent. I would never give this junk to my horses as I do not feel it is safe and my vets agree (we live in Florida). Do you have a problem with your horse ingesting sand? Does your horse live in a sand lot with no grass or hay? Have you done a test on the poop to see if there is excessive amounts present? Actual cases of sand colic are very very rare. Psyllium produces a lot of gas in some horses and gas colic is very painful. Just randomly giving crap to your horse because everyone else does is not a wise thing. Hopefully, these horses are not on over grazed lots. There is no need for all horses to be given this. It just does not may any sense, besides being an unwise choice. Do some research. Talk to a professional, like your vet. Don't be another cow in the herd following a blind bull.