I have a 25 year old QH that is prone to sand colic, he went through a stage where he had severe water-like diarreah and still to this day he will get severely runny poop. We never found out the exact cause (we tested for kidney failure, worms, bacteria, etc.) I switched him to equine senior (he was around 16 at the time). It helped some, and I also put him on monthly sandclear, which is a selenium product, if you don't have sand you can pretty much scratch that off. We also tried pennicilian and plain yogurt. I'm not a vet so my explanation might not be 100% accyrate, The yogurt has a live bacteria in it that helps with the runny poop and possible ulcers. I'm not sure what exactly cleared it up but I would try the yogurt and as someone else said, the beat pulp. Hope that helps.