Plants, fish and snails feed off the algae. My pond goes through its phase at the beginning of each spring. The air starts to warm, the sun starts to shine and the algae starts to grow. It's just a phase that a pond goes through. Soon enough the snails come out and start feeding on the algae, the lilly pads start to cover the pond surface, the pickeral rush starts fill in and life is good again. .
Nature has a funny way of making balances.
We have lost fish to everything from neighborhood Cats , Raccons, Dogs, Cats, Birds of Prey and yes, even a 3 foot Blue Heron. It took this one a few days to get up the nerve to stroll past the bedroom window with my cats watching him, but stroll he did, into my pond he went, into his mouth my fish went. Again, I'm happy that we only used cheap goldfish and not Koi. This depends on your location, depth of pond, do you have a waterfall, how much water do you move, do you have fish, etc. It may freeze up some around the edges but as long as it' s moving, it won't freeze all the way.