4 types of macroalgae that can help keep your phosphates and nitrates down

Looking to reduce phosphates and nitrates in your saltwater aquarium? Macroalgae can be a powerful tool for nutrient export, but understanding how and why they work is key to long-term success. Macroalgae absorb phosphates and nitrates from the water column as they grow, incorporating these nutrients into their tissues. When you harvest and remove macroalgae from your tank, you are physically exporting those nutrients—helping to keep levels in check. However, if macroalgae are left to die and decay in the tank, the nutrients can be released back into the water, undoing their benefits. Regular pruning and removal are essential for effective nutrient control. Macroalgae play a role in both the nitrogen and phosphorus cycles in marine aquaria, but they should be used as part of a broader nutrient management strategy, not as a sole solution.

Halimeda

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 aquarium conditions. It is relatively hungry for light (thriving under moderate to strong levels of light). One thing that makes this macroalgae 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.

As Halimeda grows, it absorbs phosphates and nitrates from the water, locking them away in its tissues. To permanently remove these nutrients from your system, you’ll need to regularly prune and discard portions of the algae. Halimeda is relatively easy to prune, is not invasive, and won’t be eaten by most aquarium inhabitants.

Chaetomorpha

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.

Chaetomorpha is especially popular in refugiums because it is non-invasive and easy to harvest. As it grows, it absorbs nutrients from the water, and regular removal of excess Chaetomorpha is necessary to export those nutrients from your system. If left unpruned, old or dying sections can break down and release nutrients back into the tank.

Caulerpa

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.

This is a picture of a fern Caulerpa macro algae available at one of the local fish stores in my area.

Ulva

Another great saltwater aquarium macroalgae that can help keep your phosphates and nitrates down is called Ulva, or sea lettuce. Ulva 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 these algae in your tank. According to some sites, it removes phosphates and nitrates from the water.

Ulva, like other macroalgae, absorbs nutrients as it grows, but it also serves as a supplemental food source for herbivorous fish such as tangs and rabbitfish. If your fish graze heavily on Ulva, its ability to export nutrients may be reduced, as less biomass is available for removal. To balance nutrient export and feeding, consider growing Ulva in a refugium and periodically harvesting it to feed your fish. This way, you can provide fresh macroalgae as a treat while still removing excess nutrients from your system.

Where to grow your macroalgae?

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.

• Display Tank: Growing macroalgae in your display tank can add visual interest and provide natural grazing for herbivores. However, some species can become invasive, outcompete corals for light and space, or release detritus that clogs pumps and filters. Pruning may be more challenging, and you may need to manage the spread to avoid unwanted overgrowth.
• Sump/Refugium: Growing macroalgae in a sump or dedicated refugium isolates it from the display, making pruning and maintenance easier. It also creates a safe haven for microfauna like copepods, which can benefit the entire system. Refugiums allow for more controlled lighting and flow, reducing the risk of macroalgae spreading where it’s not wanted. However, you won’t see the macroalgae in your main tank, and you’ll need to ensure adequate flow and lighting in the refugium for optimal growth.

Choose the location that best fits your goals, tank setup, and willingness to manage maintenance and aesthetic trade-offs.

Common mistakes to avoid with macroalgae

• Neglecting regular pruning: Failing to harvest macroalgae allows old growth to decay, releasing nutrients back into the tank.
• Overreliance on macroalgae: Macroalgae are a helpful tool, but they should not replace other nutrient control methods such as protein skimming, water changes, and responsible feeding.
• Inadequate light or flow: Most macroalgae require moderate to strong lighting and good water movement to thrive.
• Introducing pests: Wild-collected or poorly sourced macroalgae can bring in hitchhikers, pests, or unwanted algae species.
• Insufficient biomass: Too little macroalgae will have minimal impact on nutrient levels; allow time for growth before expecting significant results.

Getting started: Sourcing healthy macroalgae

If your local fish store doesn’t have a nice selection of macroalgae, you can get some online at LiveAquaria or even eBay. When purchasing macroalgae, look for:

• Bright, healthy color with no signs of decay or bleaching
• No visible pests, hitchhikers, or nuisance algae
• Species appropriate for your tank and legal in your area

Quarantine new macroalgae when possible to prevent introducing unwanted organisms to your system.