We love to care for and watch the birds at Bass Lake. We have a bird bath for them which we take care to keep full of fresh water throughout the year.
It used to be a pain to keep the thing clear with wire brushing it ever so often but a couple years ago I tried this trick. I put about a one foot length of copper tubing on the bottom which somehow seems to keep it really clean.
Give it a try. I haven't had to wire brush it since placing that tubing in our bird bath.