To the OP,
Did you treat all of your chickens for parasites or just the sick one? Did all of them get the antibiotics?
If yes, then the runny poops will cause more need for water replacement and it might not be a bad idea to give them some poultry electrolytes in their water. Also, when you're finished with a run of antibiotics, it's good to give chickens some probiotics to replace the bacteria. This can be done with plain, non or low fat yogurt, about a tablespoon per bird. Or you could sprinkle some acidopholous (sp?) powder on their food.
If you only treated one chicken, then it's quite likely that the other birds have worms and/or cocci as well and will need the same treatment. It might be wise at this point to do a complete bedding change.
Like someone mentioned, whenever there's an animal drinking excessive water, look for too much salt in the food or kidney problems.
I think that sometimes too much calcium at an early age can cause kidney disease. Someone wiser will no doubt correct me if I'm wrong.
Good luck,
Mary