I have a concrete bird bath and quite often it turns green, does anyone know if there is anything I can put in it to stop it happening again without harming the birds
no sadly there isn't. You might try putting some duck weed in as it will take the nutrients out of the water and limit the algae growth but if you do that, don't let it dry out and take the duckweed out before you empty it to scrub, replace when you refil.
If only they bathed in it, Canary. I have 4 bird baths that I clean out every day and when the squadron of young starlings arrive I have to refill them in the afternoons.
I have six bird baths and each of them has water tipped out and a little scrub with a hand brush and re-filled every day.
I love watching all the birds having a bath. It's a joyous thing.
I spent a while this afternoon watching a Blackbird having a lovely bath, then it sat near me on the lawn in the sun with both wings spread out drying off. Bliss.
I agree with replenishing the water regularly as the most important task. I use a high pressure hose on mine, just give it a good blast til the water turns clear.
Siting the bath in a position, so that it doesn't get the sun for most of the day may also help.
I think some pebbles in the bath is a good idea too.