Will malachite and formalin?
First, the good news: Malachite green and formalin won’t have any dramatic effects on the amphibians. In fact, these are recommended as treatments for fungus in amphibians. Also, the “threat to the food chain” suggested by the other user is fairly contrived, as algae, most bacteria, and any zooplanktonic organisms will tolerate theraputic levels of the drugs, and survive. Now the bad news: Malachite green is light sensitive. Exposure to light degrades it to a useless compound. That makes it a poor choice for ponds, since there is obviously light involved (unless you have a tarp you can put over the pond). Because of this, I’d strongly suggest you reconsider, and either treat with straight formalin (which is still effective against parasites and fungus), or switch to copper sulfate (also effective against parasites and fungus, but is more toxic to bacteria).
