Most commercial algaecide products are perfectly safe to use with fish, as long as they’re labeled for that use. Some can’t be used with edible fish but are safe for koi and other decorative types. Barley straw is also safe and won’t harm the fish as it slowly works to reduce algae levels. But as that sentence hints at, it’s no quick fix. You’ll have to wait a few weeks at least to see algae start disappearing after adding the straw. More fish that specifically eat algae can help, but if you already have carp or koi, you won’t see much more of an improvement from this alone.
Avoid any homemade remedies for algae that claim to be safe for fish. Vinegar and baking soda are both commonly listed as ways to treat an algae filled pond that won’t harm the fish. Unfortunately, both are actually quite dangerous for sensitive fish like koi. Stick to tested products guaranteed as fish safe to protect your investment. Any money you’ll save by using a homemade remedy for algae will quickly be lost if you have to replace so much as a single koi.
