Every once in a while for no apparent reason, Tink gets soft stools ick.
The thing that works best is to give a teaspoon of pumpkin in the morning for 3 or 4 days. Stools firm up and you don't have to change their food. My daddy taught me that one.
Course, if the stools don't form up, then you've got a bigger problem and will have to fuss with diet. But I'd try the pumpkin first. Canned - just keep it in the fridge.