Macroalgae is sometimes called the "good kind of algae" as opposed to microalgae, (i.e. Hair Algae). Saltwater aquarists have found that a number of macroalgae species are very useful in their systems, providing food for the herbivores as well as reducing phosphates (PO4) and Nitrates (NO3) levels in their tanks. Some types of macroalgae also make excellent breeding grounds for copapods and amphipods, which are a staple food for certain fish and invertebrates kept in marine aquariums.
Macroalgae cultivated in marine aquariums serves 3 different purposes: ornamental, nutrient (NO3, PO4) reduction and fish and invertebrate food.
Macroalgae Cultivated for Fish Food
Without a doubt, your best (and least expensive) way to obtain some amphipods and copepods to seed your refugium with is from a friend's tank or from a friendly LFS. Most copepods and amphipods that show up in a marine aquarium have hitchhiked in on live rock. They are easy to collect from a filter and transport to your tank very well.
If you can not find amphipods or copepods locally, here are some which are commercially available.