Greenwood wrote:

QUESTION: "I have a large concrete bird bath that has a red algae
film. I can empty the bird bath and clean it out, but the red algae
comes right back. Is there a way to get rid of it without harming the
porous concrete or the birds that drink from it?" - Jan Cook

ANSWER: It's almost impossible to prevent algae, but you can control
it. Wash out the bird bath with a solution of bleach and water, using
a "pot scrubber" brush and be sure to rinse it thoroughly three times.
I would imagine that your bird bath is in full sun so that will create
more algae then if it is in the shade. You can purchase some algae
inhibitors at your local pet store but check the label to be sure it
won't harm the birds.

A nonporous coating should be applied to the interior of concrete
birdbaths such as epoxy type concrete paint (available at any of the
major hardware emporiums), or simply insert a glazed pottery birdbath
into the concrete birdbath. In any event never ever use chlorine
bleach or any kind of soap/detergent on birdbaths... to treat algae/
mold periodically clean with a 4 : 1 water white vinegar solution and
rinse well with plain water, same as is recommended for cleaning
hummingbird feeders.