I throw vinegar or bleach in mine, often. The bleach dissipates, vinegar too. The birds aren't bothered.

Today I put 3 pennies in mine. It had some old algae, etc., in there that I hadn't tried hard enough to scrub out - now bits of it are coming off and floating! Just because of the pennies.

The pennies have to be dated prior to 1982, though. After that date, pennies don't have a high enough copper content.

My fountain is a bowling ball covered in $15 worth of pennies. Many of them HAVE to be prior to 1982...I get plenty of gunk growing in my water. None on the ball...

I use a spa treatment every once in a while. Not often enough, I guess, because I have some algae too.