I was told by an Olympic Veterinarian that Psyllium only does its job if you feed approxm. 10 full tablespoons per day to a Warmblood horse, so that is about 5 times more than the label will tell you. You can do that for a week combined with wheat bran and 1/3 to 1/2 gallon of mineral oil to get sand out of the intestines.
No longer than a week, because this treatment will have impact on the absorption of important vitamins and minerals of the horses body. Do the manure/water bucket test as mentioned in another post to find out if sand has really caused the colic and not a totally different reason like worms or gas colic etc.
In my 30 + years of experience with horses on the highest Olympic levels it has always been helpful to have Banamine handy orally or IM. If ithe horse responds positiv to it, you can feed the above mentioned mineral oil/bran mixture. Add psyllium if you suspect a sand colic, it will not do any harm if used like mentioned above, however be Sherlock Holmes and find out the real reason why the horse is colicy.
Seems this sweet pony is on the right way and hopefully it will stay like this. Good luck to everyone dealing with a problem like this.