To keep algae from growing in our cattle and dogs water we put a piece of copper in the tank. Will the same work for chickens? We do change it every morning, but we have noticed that the tray gets a little slimey.
I never heard of copper keeping water from getting slimey. I guess it wouldn't hurt. And knowing chickens curiosity, they might end up drinking more water pecking at the pennies.
What I do is Poultry Protector and/or tiny amount of bleach in the water. If you do a search on BYC for both of these "ingredients", you'll find information about them. I've been told to add more bleach than I do, I don't want to kill my chickens because the bleach is bad for them or they don't like the taste and die of dehydration so I try not to add a lot, just enough to clean & purify the water.