What can be put in bird bath water, to keep it from turning green?

Copper pennies from 1982 or before ONLY will keep the green away, not so much the black. I use 2 or 3 in mine.

Best to change it every day if the sun shines on it.

I rinse, clean and refill every day. I use vinegar to clean with too. Then dump out the water every night on nearby plants that need a refresh. Fill in morning.

https://garden.org/ideas/view/flaflwrgrl/1385/Apple-Cider-Vinegar-in-Bird-Baths/

Mine is heavy cement. I add bleach to the water and add water if below stain line. Wait 5 minutes (standing guard), then rinse well with the hose and refill with clean water. This makes it look brand new.

i use a pet/animal safe algae killer. Any aquarium store should have.

Thepondguy.com has a bleach free solution that is animal safe, order online

"The existence of algae in a birdbath is common, especially since algae spores can be transferred or deposited into your birdbath by the wind, bird feet, or even from nearby trees." The site offers 3 ways to prevent algae from growing in your bird bath. Good luck! http://www.wikihow.com/Keep-Algae-from-Growing-in-Bird-Bath

I'm going to try today. Thanks

Well done you, I hope the birds keep visiting .

Scrub the algae out of the birdbath. Leave a bleach and water mix in it for a couple days to kill any roots the algae may have grown into the birdbath. Rinse well. Fill. THEN clean it every week.