Cultivating Macroalgae in Your Saltwater Aquarium

Seaweed (macroalgae) under the water

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Many saltwater aquarists cultivate certain species of macroalgae either in their main tanks or in refugiums and sumps. Macroalgae is a major food source for a number of both fish and invertebrates in the oceans and many marine aquarists have found that growing their own macroalgae is a great way to feed their tank critters with this inexpensive natural food.

Macroalgae is sometimes called the "good kind of algae" as opposed to microalgae, (e.g., 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 phosphate (PO4) and nitrate (NO3) levels in the aquariums. Some types of macroalgae also make excellent breeding grounds for copepods and amphipods, which are a staple food for certain fish and invertebrates kept in marine aquariums.

  • 01 of 07

    Purposes of Macroalgae

    Seaweed under water

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    Macroalgae cultivated in marine aquariums serve three different purposes: ornamental, nutrient (NO3, PO4) reduction and food for fish and invertebrates.

  • 02 of 07

    Macroalgae Cultivated for Food

    Translucent red algae

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    • Kelp on Rock, Smooth Leaf (Haliptilon sp.)
      • Color can vary from a dark pink to deep red.
      • Does not tolerate high NO3 or PO4 levels
      • Does best in moderate to strong lighting.
      • Best when grown in a refugium, rather than the main tank.
    • Lettuce Algae (Ulva fasciata, U. expansa)
      • Readily accepted by herbivorous fish.
      • Removes both phosphate and nitrate from the water.
      • Use moderate to high lighting and keep it in an area of high water flow.
    • Limu Manauea (Gracilaria coronopifolia)
      • Endemic to Hawaii.
      • Excellent surgeonfish food.
      • Removes ammonia, nitrate, and phosphate from water.
      • Does best in medium light.
      • Under the right conditions, biomass can increase 150 percent per month.
    • Prickly Sargassum 
      • Has short thick blades with prickly margins and spherical gas bladders.
      • Endemic to Hawaii.
      • Great surgeonfish food.
    • Feathery Caulerpa (Caulerpa sertularoides)
      • Does best in medium light
      • Likes a strong to medium current
      • Great surgeonfish food
  • 03 of 07

    Nitrate and Phosphate Reducing Macroalgae

    Caulerpa Seaweed, Caulerpa sp., Susac, Dalmatia, Adriatic Sea, Croatia

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    • Shaving Brush Algae (Penicillus sp.)
      • Will benefit from the regular addition of an iron supplement, as well as trace elements.
      • Helps reduce nitrate (NO3) and phosphate (PO4) from the water column.
    • Spaghetti Algae (Chaetomorpha sp.)
      • This algae is fast growing and is not palatable to many herbivorous species.
    • Caulerpa
      • Must be pruned regularly to prevent rapid die-off during sexual reproduction, which will pollute the tank.
      • Can be found in various shapes, including flat blades, feather blades, notch leaf, and a grape-shaped variety.
  • 04 of 07

    Ornamental Macroalgae

    Halimeda Ghost Pipefish mimic macroalgae

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    • Shaving Brush Plant (Penicillus sp.)
      • Will benefit from the regular addition of an iron supplement, as well as trace elements.
      • Can go a long way towards helping reduce nitrate and phosphate from the water column.
    • Halimeda Algae (Halimeda sp.)
      • Does not tolerate high NO3 or PO4.
      • Requires calcium.
      • Does not tolerate extensive pruning.
    • Kelp on Rock, Smooth Leaf (Haliptilon sp.)
      • Color can vary from a dark pink to deep red.
      • Low nutrient plant.
      • Does best in moderate to strong lighting.
      • Best when grown in a refugium, rather than the main tank.
    Continue to 5 of 7 below.
  • 05 of 07

    Ornamental Macroalgae (Cont.)

    Caulerpa lentillifera (umi budo) in hands

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    • Maiden's Hair (Chlorodesmis sp.)
      • Good breeding ground for amphipods and copepods.
        • Contains a toxic substance that deters herbivorous fish from eating it.
    • Mermaid's Fan (Udotea sp.)
      • Does not tolerate high NO3 or PO4.
      • Requires calcium
    • Caulerpa
      • Can be found in various shapes, including flat blades, feather blades, notch leaf, and a grape-shaped variety.
      • Must be pruned regularly to prevent rapid die-off during sexual reproduction, which will pollute tank.
  • 06 of 07

    Nutrients for Macroalgae

    Algae In Lake

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      • Some algae require phosphate and nitrate to survive.
      • Some algae will not tolerate high nitrate or phosphate.
      • Calcareous macroalgae (e.g., Halimeda) require calcium supplements.
      • Some macroalgae (e.g., Shaving Brush) benefit from the regular addition of an iron supplement, as well as trace elements.
  • 07 of 07

    Growing Macroalgae

    Macroalgae species selection is mostly dependent on two factors: 

    • What species are available in your local area;
    • What do you intend to do with the algae: decorative, grow in refugium for water purification, or for fish food?

    Most macroalgae require medium to strong lighting to thrive. The color of the macroalgae may vary with different light intensities. Some macroalgae can change the pH of the water due to their respiration, so you should regularly monitor your aquarium water quality. They produce oxygen in the light, but absorb oxygen and produce carbon dioxide, which lowers the water pH, in low light conditions.

    When you have a large sample of a macroalgae, make cuttings to attach (specifically the holdfast, if the algae has that) to a shell, live rock or coral skeleton with super glue. Place these new cuttings in the aquarium with proper water quality, good lighting, and good water circulation.

    One thing an aquarist should know about macroalgae is that it may reproduce sexually, releasing spores into the water, resulting in extra nutrients that may raise nitrate and phosphate levels. Poor water quality will more likely stimulate the algae to reproduce sexually and release spores.

    Good Beginner Macroalgae to Grow

    Codium is a green macroalgae that can be attached to your live rocks or shells with super glue. Codium does best with moderate to high circulation after it has been secured in place. This particular macroalgae does not have strong lighting needs.

    Caulerpa is a large genus of macroalgae that grow quickly, and can overrun your display tank without proper trimming. Because of this, they are a better candidate for the refugium, unless you can put in the effort to keep it under control. The Grape Caulerpa and Cactus Caulerpa are less likely to do this than the rest. Caulerpa is easy to grow, requiring only 2-3 watts of lighting per gallon. The runner system can be easily cut to start new plants, and it can be split into multiple plants with ease. It should be attached with super glue or fishing line to a rock or hard substrate to keep it from floating. In a short amount of time it will attach to the rock, and then the fishing line can be removed.

    Halimeda is a popular decorative macroalgae because it will not be eaten by herbivorous fish or invertebrates due to it calcified nature. Members of this genus can be good indicators of the balance between your magnesium, alkalinity and calcium levels, all of which will affect the growth rate of this algae.

    Ulva is an easily cared for green macroalgae that can survive extended periods of darkness and fluctuations in water temperature. However it will do best under 3 watts of illumination per gallon. Glue or tie it onto rocks or shells to get the holdfast to attach.

    Chaetomorpha is easier to care for than other macroalgae. Simply place it in your aquarium among the live rocks and substrate, put it under 2-4 watts/gallon of full spectrum light and it will grow. Chaetomorpha works great for filtering use. Periodically harvest the Chaetomorpha to remove accumulated nutrients from the water as it grows.