For your 50 gallon marine aquarium you want about 50lb of live rock. In addition to cycling out ammonia and nitrite, live rock supports anaerobic nitrifying bacteria that will consume nitrate. This is important as marine fish are much less tolerant of nitrate than freshwater fish, and you want to keep the nitrate much lower than would be the case for a freshwater aquarium. Zero nitrate is really the goal, whereas in a freshwater aquarium 20ppm + is usually acceptable.
Are you keeping any coral?
Also, due to the nature of ammonia toxicity, (ammonia is more toxic at higher pH), you have to be really careful of ammonia as the pH of a marine aquarium will be up above 8. Even 0.5ppm ammonia is pushing the limits of where ammonia gets toxic in marine aquariums.