Yes, agree with Coraline about feeding wet food with a high meat content, or a balanced raw diet, definitely no dry (or just a small amount as a treat maybe - but again go for a dry that is high meat/low carb, such as Applaws or Orijen).

Cats are meant to eat the carbohydrates that are in most dry foods and some wet foods - persians in particular are intolerant of carbs - mine were, and now they have mainly raw with some of the wet foods that Coraline mentioned.

I also agree about just getting them checked over again at the vets - to rule out parasites etc and he might recommend a probiotic supplement that could help with the diarrhea.

When you make any food changes remember to do it gradually to avoid tummy upsets.