Metronidazole uses ions to pass through the fish's membranes. Saltwater does have plenty of ions so Metro is certainly effective in saltwater. But Freshwater does also have ions, though less amounts than saltwater. The amount of ions present in Freshwater aquariums varies depending on the water. Most freshwater aquariums do have enough ions for Metro to work, unless your water is really soft. Of course, the more ions, the more likely Metro will be effective. Most freshwater does have a good amount of ions unless your water is really soft. Many people have used MetroPlex in freshwater aquariums and it has worked (but again it does depend on your water).
MetroPlex label even still says "Marine & Freshwater". Aquazole is guaranteed to work because it includes Potassium Sulfate (P2SO4) which act as the necessary amount of ions for adequate absorption. Yep, the same Potassium Sulfate people use to fertilize their plants. If you have that on hand, you can simply dose some of that along with MetroPlex and it is the same as Aquazole. Aquazole just includes Potash Sulfate, to makes sure enough ions are provided for the med to work. if you already have enough ions in your water, MetroPlex is fine.
You can be on the safe side though and get Aquazole (not sure how it equates, price to dosages though). Or you can feed Metronidazole medicated fish foods to the fish, that method bypasses the need to pass through the fish's membranes via the dosed water column. Or you can just use PraziPro (though that doesn't as much as Metro does).