It didn't harm my coral, hermits or snails. The bottle says it doesn't harm inverts. It is mostly melaluca oil. A natural tree extract. Here is the description from the company. I know we can't link the site, but I don't think anyone will object to the description.
MelaFix
Contains the natural botanical extract from the Tea Tree (Melaleuca), an excellent alternative to resistant strains of bacteria that are unaffected by traditional medications. Treats bacterial infections such as red ulcers, fin and tail rot, cloudy eyes, mouth fungus, and others in as little as 4 days. Also heals open wounds, ulcers, and damaged fins. Doesn't affect pH; safe for invertebrates. NEW MelaFix Marine can also be used with corals to control bacterial infections, reverse coral degradation and remove parasitic flatworms. 8 oz treats 480 gallons of marine or freshwater. 64 oz is a concentrated formula and treats 19,000 gallons. 8 oz MelaFix Marine treats 474 gallons marine water.