I soak my birdbath in a 10% bleach solution for an hour or 2 then scrub with a stiff brush and rinse thoroughly. I also spray it to refill and clean it out whenever I have the hose out to water my plants. Sometimes a couple of copper pennies placed in the bottom will help to deter or slow the algae growth.
Elbow grease will be your best tool. Soak the red stuff a bit in plain water if the bath has dried, then use a good strong scrub brush to scrub it loose. (It is a form of algae.) Rinse well, and scrub again, if necessary. If you can empty and refill the bath daily, or even every other day, you can stay ahead of the algae; and the birds will appreciate the fresh water!
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