Copper-Based Bottom Paint - The copper in antifouling bottom paints serves as a biocide to keep hard growth like barnacles and zebra mussels from attaching to your hull. Typically, the more copper a paint has, the more effective it'll be - and it'll also cost more. The copper comes in two forms: cuprous oxide and cuprous thiocyanate. Cuprous oxide is a darker copper, and results in bottom paint colors that are darker and less vibrant. Cuprous thiocyanate (also called white copper because of its color) results in brightly colored bottom paints.
Copper-Free Bottom Paint - Copper-based bottom paints are effective, but they're harmful to fragile marine ecosystems. Environmental regulations in some states restrict the use of copper-based bottom paints. Copper-free biocides include zinc pyrithione and Econea™ (tralopyril). As with copper biocide, Econea prevents hard shell fouling from barnacles and mussels. The algaecide zinc pyrithione prevents fouling from weeds, grasses, and slime. Econea is more environmentally friendly because it breaks down quickly into a biodegradable form that is not harmful to marine life. Another advantage is that, compared to copper-based paints, less Econea is needed to prevent founling - bottom paints made with just 6% Econea are as effective as bottom paints with a copper content of 50%. In addition, copper-free bottom paint doesn't leave a green patina at the waterline like copper-based paints can.