I know that when I used to breed fish and had a house full of fishtanks, any exposure whatsoever to sunlight causes algae to grow in fishtanks. The tanks would have to be positioned such that they had no exposure to sunlight through windows. I would imagine it would have that same effect in a bowl of water exposed to sunlight, as well.
Short of excess light being an issue, it could be the water from your tap. Our dog's water would get slimy and occasionally a bit of algae in her water bowl, even though she emptied out her bowl at least once a day. Someone else also suggested to us it could be our tap water. We bought a new Brita pitcher and are using the old one now to filter the dog's water, and it no longer gets slimy, filmy, or gets algae in it in any way ever since we started filtering it (in February of this year).
So maybe your tap water is contributing to the algae issue.
Short of excess light being an issue, it could be the water from your tap. Our dog's water would get slimy and occasionally a bit of algae in her water bowl, even though she emptied out her bowl at least once a day. Someone else also suggested to us it could be our tap water. We bought a new Brita pitcher and are using the old one now to filter the dog's water, and it no longer gets slimy, filmy, or gets algae in it in any way ever since we started filtering it (in February of this year).
So maybe your tap water is contributing to the algae issue.