Here's a thread with recommendations for dosage: https://www.backyardchickens.com/t/78403/apple-cider-vinegar. I'm not terribly scientific in how I administer it - I usually just put a splash of vinegar into a gallon bucket (and scale up for greater volumes).
I wouldn't count on the ACV to stop algae growth...I had a problem with algae growth accelerating during the summer when I put vinegar in waterers; I think the heat/humidity was responsible. Once the weather cooled off, the algae died off, even when I added vinegar. There's discussion in this thread: https://www.backyardchickens.com/t/624533/mold-in-my-water-with-acv.