During the hot summer months, I would clean the bowl every day and give the dog fresh water. To make it easy, just take a pail of fresh water and a cloth with you when you go out to feed the dog. Keep the dog food dish clean too. I'm sure the dog would notice and appreciate it.
You have to wash your dog's water dish every day to keep it clean. Animals enjoy clean fresh water in a clean water bowl. Don't forget the dog's saliva is being secreted as he drinks water which causes the slippery film in the bowl.
When the bowl starts to turn green, fill it with water and add about a quarter cup of chlorine bleach. The bleach will kill all the algae. Give the dog another bowl to drink from while this is happening. Be careful where you dispose of the water. I have found from experience that if you dump it on the ground it will kill all the earthworms in that area!
Wash it every day whether it has algae in it or not; it's not that big of a deal to do. How would you like drinking out of the same dirty glass every day? Think about it; not only is there saliva from the dog drinking out of it with its tongue (which bacteria will grow from) but it's also outside where dust, bugs, etc are landing in it.
Try using a different bowl. My vet says no plastic or metal for my dogs. I use an old crock pot insert and I wash it daily. That green stuff is algae, it does not form in one day. Before you fill it... wash it.
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