I change the water in our birdbath every 3-4 days. To keep moss from building up in the bowl, I brush the bowl good and then tip it up and slosh the dirty water out. Using the hose, I spray the remaining dirty water out and fill it with fresh water. To have the brush handy, I keep it on top of the hose reel.
We've had this birdbath for about 10 years and this is my first year keeping it clean with the brush and it is working very well.
By littergitter from NC
Check out these photos.
I have a lot of bird baths and most of them are made from cement and stone. They are very heavy and my biggest pet peeve about them is that they sit in the sun and they turn green with algae.