There are hundreds of claims of homemade mixtures for killing algae, but baking soda is not one of the effective treatments you should try. Since baking soda is highly alkaline and reacts when mixed with an acid, many people assume it can be used to change the pH of a pond to discourage algae growth. However, even a large amount of baking soda dissolved in a pond will have little to no effect on either algae or pH. You’ll have to add so much baking soda that any algacidal effect will come from the salination of the water instead, which also kills all other plant life and fish. Then you’ll have some extremely salty water that must be disposed of in a sewage system since it will damage any soil it’s poured over.
Baking soda should only be used in the pond for scrubbing stains or algae deposits on an exposed pond liner while the water is drained. A small amount left behind from this kind of cleaning won’t hurt any fish in the water or have a strong effect on the pH. If the pond is filled and having problems with algae or pH swings, use products sold by pond supply stores that are rated as safe for fish and plants rather than experimenting with products from your cupboard.
