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 pre-1982 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 except when the temperatures stay above 90 for extended periods. Copper is not safe for fish.