Here are 4 types of macroalgae that can help get your phosphates and nitrates down:
Halimeda is a macroalgae that you can actually use as a decorative plant. It is generally hardy in a saltwater tank, growing well under most appropriate aquarium conditions. It is relatively hungry for light (thriving under moderate to strong levels of light). One thing that makes this macro algae interesting (could be considered a downside by some) is that it absorbs calcium from the water and includes it into its tissue, similar to coralline algae.
It is relatively easy to prune, is not invasive and won’t be eaten by most aquarium inhabitants.
Chaetomorpha is a thin, spaghetti-like macroalgae that grows quickly in a saltwater aquarium. Very quickly. Give it moderate-to-high lighting and good water flow and it will do a great job of removing nitrates and phosphates from your aquarium water. It grows densely in clumps that also serve as a good home for copepods and other invertebrates like mysis shrimp.
Caulerpa is a macroalgae that you either love or hate. Love it, because it is extremely tolerant of even less-than-perfect aquarium conditions and grows…well…like a weed. Hate it because it grows…like a weed and is hard to get rid of…like a weed.
Another great saltwater aquarium macro algae that can help keep your phosphates and nitrates down is called Ulva, or sea lettuce. Alva is a fast-growing, hardy macro that will grow well in most tanks…that is…until it gets eaten by your tangs…which is another great reason to try and grow this algae in your tank. According to some sites, it removes phosphates and nitrates from the water.
If you like the look of it, you could grow the macroalgae in your display tank. If you want to lower phosphates and nitrates without having to look at the macro, then the best place to grow it would be in your sump or refugium.