Macro Algae provides many benefits in the saltwater aquarium.
One of these benefits is to provide a food source for the inhabitants of the aquarium. Another benefit macro algae provides is a means of nutrient export for phosphate, nitrate, and nitrites. Macro algae can also provide a place for copepods, mysis shrimp, and smaller fish to hide.
Halimeda, Chetomorpha, and varies types of Caulerpa are the most common macro algae used in the home aquarium.
Halimeda which is also called money plant grows in small round links and adds calcium to the aquarium when it dies. It also uses calcium when it grows and is not as good for nutrient export because it does not grow as fast as other types of macro algae. It is an attractive macro algae and its growth in the aquarium is easily controlled.
Caulerpa is a very common algae used in the saltwater aquariums. It is also called Grape algae, Razor algae, and Feather algae which come from its appearance. This algae is great for nutrient export and many fish in the reef aquarium will eat varies types of it. One problem with this macro algae is when it goes asexual it releases a cloud of spores into the aquarium and then dies. In doing this it also releases the nutrients it absorbed back into the saltwater tank. The growth of Caulerpa is also harder to control in the aquarium. It attaches to anything it can and frequently breaks apart when trying to remove it and the pieces left behind can quickly grow back.
Chetomorpha is one of the best macro algae for general use in the marine aquarium. Common names for this macro algae are brillo pad and spaghetti algae. The names come from the macro algaes wired tangled appearance. It can grow fast for nutrient export and does not go asexual as often as Caulerpa. It is also liked as a food source for a lot of marine fish. Although it is great for filtration you would not want to use it in the main aquarium. It is a free floating algae and will get caught in powerheads, corals, and many other places you don't want it.
When macro algae is used as filtration in the sump, it needs to be given 8 to 12 hours of light on the reverse cycle of the main tank. This will help keep ph levels more stable by increasing oxygen levels in the sump at night when the main aquarium is giving off carbon dioxide. Carbon dioxide given off by fish and macro algae at night can create ph swings in the aquarium and by running the sump on the opposite lighting period will help prevent this.
The lighting for the algae used in the sump for filtration should be in the range of 5000k to 10000k. This light range is closer to what you find in grow lights and is best for growing macro algae quickly. Giving the algae a rest period of darkness will also help it grow quickly.
As the saltwater algae grows, portions should be removed which is a means of nutrient export. You can then can trade it into the local pet store for credit or feed it to the algae eating fish as a treat.