Why are marine fish so much more brightly colored than tropical fish?

Answer 1

Probably a lot of the reason is from their habitat. Marine fish from near-surface coral reefs have colorful corals, sponges, algae, and the like, so they didn't have to adapt to blend in to a "drab" background as fish that are found in lakes and rivers.

There may be more venoumous and toxic species as well in saltwater, and the bright coloration serves as a warning to potential predators. So, of course mimics will arise that have similar coloration, but are harmless.

But Dee is right that not all saltwater species are colorful. As the depth of the water increases, there is less light, and red wavelengths dissapear quickly, so as the depth increases, the fish tend to be more neutral (brown, gray) or toward the blue end of the spectrum. Just the same, there are some fish found in freshwater that can be pretty colorful. Google some photos of sunfish (pumpkinseed, longear sunfish) or male darters (rainbow, cherry, firebelly, sharphead).

Answer 2

Actually, the majority of marine fish are not brightly colored...it's just that the ones appropriate for an aquarium are more brightly colored than the rest of the species of marine fish.