I put a few snails and crayfish into my outdoor ponds. The snails are a real help with the algae, but I try to have ~70% of the surface covered with vegetation by mid-summer. This helps block out much of the sun and keep the water temperature more stable. The water plants also consume some of the excess nutrients required for algae development. Trapdoor snails (livebearing) are great for cleaning the bottom and sides. Melanthe snails (egg layers) mostly float and consume algae. I have two Apple snails that I overwinter along with my goldfish and Koi. They go into the ponds too, but you have to watch. They mostly eat detritus, but they love to eat lilies and other plants too. I also keep a couple of crayfish in the one pond to help out. They reproduce like rabbits in my pond, but some have witnessed them crawl away if they do not like their home. Because they can nip goldfish and create sores, they can be more harm than good.
HTH
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