To keep algae growth from your bird bath drop some pre-1982 copper pennies in. I have what I would call a normal size bird bath & I use 7 pennies. The reason for the pennies being pre-1982 is that before that year, the pennies contained copper, a natural algicide. You could also use a small length of copper pipe or tubing, or any other copper coin. I have found this to work extremely well with the exception of those times when the temperatures stay above 90 for extended periods. Copper is not safe for fish so this would not be a good thing to use in a fish aquarium or a pond that has fish in it.