Are they still being free ranged? If not, then they would have had a very sudden change in diet (no more grass, bugs, etc) and they might just be adjusting to it.
Do they have any other symptoms? Puffed up, lethargic, wheezing etc?
Do you have any other chickens? If so, are you quarantining the new ones until you can make sure they are healthy? This is very important to ensure you don't contaminate your original flock and lose everyone, if it's something serious.
I don't think that giving them antibiotics is a good idea, especially if they aren't showing any other symptoms. Different antibiotics treat different things and if you don't have a good diagnosis, you are just contributing to antibiotic resistance. They are also terrible for the digestive system and often CAUSE runny poop.
I would try to add some greens to their diet if they don't already have access to any and try giving them fermented feed. Do a search on here and you will find more information than you know what to do with. A lot of people claim that it makes the droppings more firm and less smelly.