It's important to know when to apply fertilizer to your leafy greens — and when not to. If you are treating your tank for anything, don't use the water in the garden. Whether you're treating fin rot, a parasite, or an algal bloom, adding chemicals to your tank introduces unknown substances into the water. Copper, for example, is common in treatments for algae and some parasites because it's effective and doesn't discolor the water. This advice isn't just for marine tank owners either.