I agree it's important to find out WHY she's getting the diarrhea, and how severe it is. Foals can die from severe diarrhea if left unchecked. She could need antibiotics, a change in her feed, or ???
My three-month old filly currently gets diarrhea from eating sand while turned out in the arena. It's not a mystery to me... I see her eat mouthfuls of it. Every time I see it, I give her psyllium so that's my fix for her specific problem.
Bio-sponge can help a lot for clearing up diarrhea, but an antibiotic such as Metronidazole may also need to be used from your vet.
Good luck,
Andrea