No labeling or otherwise informing the customer that meds are in use is not a bad idea. However, that is not the question you asked. You asked if an LFS should be held responsible for damages if someone dumped the bag water in their tank and lost some animals.

I've been a hobbyist alot longer than I've owned a pet store and I have lost animals because of my mistakes. I've lost animals because of wrong advice from an LFS but I didn't blame them for it. My tank, My animals, My responsibility. If someone tells me to feed my tank a quart of motor oil and I do it because I didn't know better well it's still my fault not theirs, they were just wrong. Just my view of things.