mimijacobs, you might try "Wet and Forget" eco-friendly . We used it on a concrete patio. Is it safe for bird feeders ? Read directions and call consumer number on container about any questions you may have. It appeared not kill surrounding grass.

Might try moving you bath to a more open area away from any over hanging trees. My sister-in-law was having same problem...she cleaned her bath with vinegar.

I'll look it up, thanks. That bird bath is in an open completely sunny spot. No shade whatsoever.

Mimijacobs, the sunlight is probably the cause. Seems counterintuitive, but unlike fungi, algae are photosynthetic and thrive in the presence of direct sunlight.

Try moving it to a shaded spot that is not directly under a tree...spores falling from trees can contribute too.

This is an ongoing issue for me, too. I've moved my BB to a shady location (algae needs sunlight in order to thrive) conveniently located near a garden hose. Daily water changing along with a vinegar scrub once a week has really helped.

My grandmother swears by old pennies. Hers always stayed clean, and there were always old pennies in the bottom. She says they have to be pre-1982 for the copper content.

In the waterworks we used copper sulfate in the reservoirs several years ago. It works but is expensive. You may want to look into dry chlorine tablets from a pool supply store.

There are other algaecides on the market but I'm not sure if any of them are safe for wildlife to drink.