Why is my Macroalgae not Growing?

Macroalgae are well known for their ability to grow at an astoundingly rapid pace. For example, some kelps can grow to 200 feet long from a spore in a single season. They’re called seaweeds for a reason, right? One would think that in the sheltered environment of an aquarium (especially so in the refugium) any “macro” would flourish. There are usually more than enough nutrients there for them to do great!

But, macroalgae (like any other organisms) are adapted to survive under certain types of conditions. Even if all other parameters are perfect, if just one is off kilter, than the entire crop can fail. Aquarists who venture on the cheap or idle side are most likely to make these mistakes.

Really, keeping and growing macroalgae is rather simple and straightforward if the basic needs of the plant are met. These needs are not unlike other plants: They like a lot of light and a lot of food in an environment that is free of large herbivores. So if you throw a clump of chaeto in a refugium, throw a light over it and count on it eating fish poop, all will be well? Not necessarily!

The light of their life

Let’s start with lighting. Is it bright enough and of the right spectrum? What our eyes perceive as bright (particularly when indoors viewing artificial illumination) is often not very bright at all. A plant may survive but not grow if it receives just “enough” light, an amount referred to as the compensation point. To achieve fast growth (or any growth) the gardener must apply a very powerful lighting system. This should be at least the intensity one would use to keep SPS coral.

And that’s not enough; intensity won’t matter one bit if the spectrum is wrong. Getting this right can take a bit a research, as different macros naturally occur at different water depths and are therefore prefer different spectra. Generally, red algae fare best under bluish light whereas green algae fare best under more full-spectrum (i.e. white) light. Full-spectrum lighting is generally the best choice for systems cultivating multiple types of algae.

Feed them right

Believe it or not, macros can starve in many under-maintained systems. Sure, there is way more (way more!) than enough nitrate and phosphate for active growth. But what about molybdenum? Boron? Even elements such as iodine and copper (which are potentially toxic to marinelife) are absolutely vital in trace quantities. These substances are rapidly depleted in a typical reef aquarium. Without a steady supply of certain “trace elements,” plant growth will come to quick halt. To correct these imbalances, one can add products that include a broad range of trace elements in their proper proportions. Or, better yet, stick to a diligent water change routine (most synthetic sea salt mixes include all the major trace elements).


Sometimes macros will grow to some extent, and then stop. This can occur in a very healthy aquarium system and is usually due to crowding. Crowding hurts the plants in two big ways. For one, as the algae patch increases in density, there is more and more shading (less light!). Additionally, water flow between plants can become restricted. This results in stagnant areas that reduces gas exchange as well as access to nutrients. Luckily, the solution for this is quite simple. All the aquarist needs to do is carry out some trimming! It is better to harvest older plant biomass. This material can then be thrown out with the trash or fed to herbivores (yellow tangs, emerald crabs, etc.) in the display tank. The remaining material in the refugium will again have space to expand into (room for new growth) and receive adequate light/water flow.

Other considerations

In some instances, other algae can reduce the growth rate of macros. This is due to competition. Suppose you have everything in your system just right for algae growth—a virtual algae paradise. Suppose also that there are other, less desirable forms of algae present. These too will proliferate aggressively. They might, in fact, grow a lot faster than your macroalgae. Planktonic forms can really discolor the water, stealing much of the available light. Benthic forms (known as epiphytes) can actually grow on top of the macros, blocking them from light and water flow. What to do?

In these cases, it’s virtually impossible to physically remove the offending algae. And, any effort to poison them would likely harm the macros as well. Here, the best option is ultraviolet sterilization (to control unwanted plankton blooms) and small herbivores such as hermit crabs (to control epiphytes). Be sure that your refugium clean-up crew consists of species that will consume only the bad algae and ignore the good. Moreover, purchasing only “clean” macros is a great way to start, since you’ll be bringing far less of this bad stuff home with you!

No more micro macros

Whether you use macroalgae for nutrient export or simply because you find them to be beautiful, it can be frustrating when they won’t grow. Fortunately, there are just a few things to look at as you improve their culture environment. With the right lighting, adequate water chemistry, lots of space and few competitors, your macros will grow to be huge, lush and vibrant!