Chickens, like many birds and animals, will pant when overheated or stressed. Panting changes the pH balance of the chicken resulting in poor metabolizing of nutrients, a lessening of food and/or water intake, lethargy and interruption or cessation of the laying process. A tablespoon of ACV to one gallon of water mixture will help bring the bird's pH balance back to normal levels (or near normal).
ACV will help somewhat to reduce algae levels inside your plastic waterers. If your bird's water source is a municipal water source it is probably chlorinated which will help a bit keeping the algae down. If your waterer is in direct sunlight you will probably be fighting algae all Summer long. Wrapping, painting or encasing the waterer in a dark color will help immensely.
There are trace nutrients in ACV, all helpful for our birds. I buy my organic ACV here in NC at the local Harris Teeter grocery store in the health food section...but in a pinch even regular ACV will help bring their pH balance back toward normal.
Personally, I have put ACV in my bird's water since they were chicks. I wouldn't want to suddenly offer "funny tasting" water to them in the middle of a heat wave.