It's best to dilute oxygen bleach and use it sparingly. Made with hydrogen peroxide or a mix of hydrogen peroxide and washing soda, oxygen bleach is gentler and less toxic than traditional chlorine bleach, and it's safer for the environment—without harsh fumes or the chance of creating dioxins.
But before you resort to oxygen bleach, try this technique at least once per year—thoroughly clean your dock (or deck) using a hose (a pressure washer will have too much force and splinter the wood), concentrating on the cracks between the boards. Maintenance of your dock (or deck) is also important, even regular sweeping will keep the surface free of debris, keeping many types of stains or mildew at bay.
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If manual removal of mold and mildew doesn't work, choose oxygen bleach or a product with citric acid (from lemons and limes).
And don't be overwhelmed by the store shelves full of wood cleaning products. Many contain toxic chemicals like oxalic acid, which is very corrosive.
The fishes and I thank you for being an excellent steward of your lake habitat!