I was thinking you might be able to put a narrow strip of zinc near the top and when rain hit it would carry the zinc down the side of the house without discoloring it.

But, spraying once a year might be safer.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).