It is hard to guarantee any medication as 100% safe given there are many variables to consider. Metronidazole is a much gentler treatment for "a tank then a copper, formalin, and a few other treatments. Many have used Metronidazole and have had great success treating infections with invertebrates and corals in the system. As a general rule when medicating any system it is a good idea to observe to make sure they do not show any signs of stress. Metronidazole will also be quite effect in the medicated food mix while also ensuring that as little of the medication is in the water column. If you are looking to keep medication from entering the water column then use Metronidazole in the medicated food mix with Focus, and Garlic Guard. Focus will bind the medication while adding antibacterial properties. Garlic Guard enhances the flavor, adds vitamin C, and the natural internal parasite removal properties of garlic. Putting in a food mix will keep the medication out of the water column and ensure the fish internalize the medication and better treatment results.