I'm so sorry you lost your horse, Susan!
I agree it's unlikely to be the stone dust but it's hard to be sure.
For the psyllium - you need a lot more than a teaspoon, you need a couple ounces - depends a bit on what schedule you feed it on. For my guy who had a bout of fecal water I was using it daily, 2 ounces by weight (he is about 300 lbs). For sand removal you're supposed to not feed it all the time, usually they recommend one week a month. Not sure how big your horse is but you can't really OD them on psyllium anyway (it can be a palatability issue, you'll need to soak it most likely to get them to eat the loose stuff in any appreciable amount, and then it gets a little slimy if it isn't mixed with enough other stuff). I used a pelleted product because I'm not able to soak his feed, and he ate that just fine. Not so happy for the wallet, but, horses.