Welcome to BYC. Are you located in the UK? The product you are asking about is suggested to go into plain non-chlorinated water. ACV is really not necessary, nor is using probiotics daily in your chickens’ water. It probably is not bad for them, but not necessary unless they have been on antibiotics or recently had an illness. I used to put ACV in my plastic waterers, but found the water got slimy unless I cleaned my waterers and refilled them, especially in warm weather. A lot of things get recommended for chickens that may not always be necessary. Clean water, clean food, and a clean coop can give them most of what chickens need.