It’s surprising, the number of beneficial uses of salt in your backyard pond. In fact, in safe quantities, salt can improve the health of your fish and build up defenses against parasites and harmful bacteria. Keep in mind, though, that fish benefit from and can tolerate only a minimal amount of salt, and freshwater aquatic plants can tolerate even less. For this reason, you won’t actually see a lot of professionals recommending salt as a solution for string (filamentous) algae. To make matters worse, it’s completely ineffective against the other kind of algae -- the type that causes green water.
If you’re dealing with string algae in your pond and it’s not possible to provide additional shade by adding water lilies or other floating plants, then consider using some sort of screen (a sail or patio umbrella, for example). Aside from that, your remaining choices include use of chemical products or natural (iodine-free) rock salt. Chemical products can be dangerous to apply and dangerous to your fish if directions are not carefully followed. Even when used properly, these chemicals can inhibit growth in the aquatic plants that you DO want. Salt is less toxic to you and me, but algae is a plant after all, and since your goal is to use enough salt to kill it, your other plants are also vulnerable. Finally, while rock salt applied at 1lb per 100 gallons will kill string algae, the resulting mass of dead and rapidly decaying vegetation will pollute your pond and cause even more problems with water quality. If you want to give salt a try, take the time to remove as much string algae as you can by hand before you start. Finally, once you’ve raised the salinity enough to deal with the algae, you’ll need to do some substantial water changes before you can safely return your fish and aquatic plants.
