Harnessing the power of macroalgae to defeat annoying film algae or hair algae is a common occurrence in the marine aquarium hobby. All kinds of algae have similar needs: lots of light and nutrients. Because of this, macroalgae can compete with nuisance algae, making it very desirable. Along with that, macroalgae is a great habitat for copepods and other microcrustaceans. If you plan on keeping fish such as a leopard wrasse or mandarin dragonet, having a large copepod population is important. Macroalgae can help with that as well. Although growing macroalgae in a reef aquarium is not an all agreed upon thing (very few things are), it is still very popular and desirable. It may even have benefits that we are not aware of yet!
Because macroalgae is so desirable, it can be startling when it doesn't appear to be growing. There are a lot of different causes for this, similar to there being different reasons why corals don't grow. As much as we'd like to give a cut and dry answer, we can't. However, we can go over some possible causes and things you can do.
Is it actually not growing?
If you have ever tried to lose weight or put on muscle, you know that results are hard to see day to day. You might look at yourself in the mirror everyday for a month and feel that you see no results. This is a similar thing that happens with corals and even macroalgae. If you look at your aquarium everyday, you may feel that nothing is growing aside from maybe some film algae. While it is certainly possible that something may not be growing, there is a high chance that you only feel this way because you often view your aquarium.
Unlike nuisance algae, macroalgae is somewhat slow growing. It grows much faster than corals, but not as fast as nuisance algae. If you think about it, that's a lot of the reason why hair algae and film algae are seen as annoying while macroalgae isn't. Macroalgae is much more manageable. It is actually good that it doesn't grow as fast as microalgae.
How can you tell if your macroalgae is growing or not? Well, the first step is to determine if it is dying. If the macroalgae is dying, it is certainly not growing. Dying macroalgae will look dull in coloration and it will eventually turn gray. Macroalgae that is not dying will be mostly a solid color and look full. If your macroalgae seems to be healthy, just not growing, it is at least a step ahead of dying macroalgae. To determine if your macroalgae is growing or not, only look at it once a week. By not checking it, you will be able to see a more noticeable difference. You could even take it a step further and photograph the macroalgae. Remove it from your refugium or macroalgae scrubber and place it in a bowl, preferably a white one. Take a picture now and in one week. Take a picture once a week on the same day using the same bowl. Do this for a month. If you can't see any difference between any of the pictures, especially the first and last one, then your macroalgae is probably not growing or growing slower than it should.
Causes for macroalgae not growing
As mentioned earlier, there are a lot of reasons why macroalgae may not be growing. It's like getting head aches or stomach aches; there's no one thing that causes those to occur. With macroalgae, there is a small group of things that could be happening.
Possibly the number one reason your macroalgae is not growing is because of lack of nutrients. Macroalgae, and all algae, need nutrients to grow and live. Without nitrates and phosphates (which are nutrients in case you didn't know), macroalgae will stop growing and eventually die. Now, before you pat yourself on the back for having low nutrients, let's determine what could cause your nutrients to be low. First, you should test your water using a trusted test kit. Test for phosphates and nitrates. The more accurate of a test kit you can use, the better. Hanna Instruments test kits are some of the best because they give you a number. That way there is no guessing if colors match or not. If you get a nitrate reading of 1 or above and a phosphate reading at all, low nutrients are probably not the cause of your problem. If you get 0 for both, there is a chance this could be the cause. However, a 0 nutrient test result doesn't mean there are not nutrients in your system. When there is a large amount of microalgae present in an aquarium, the microalgae can consume nutrients so fast that there is no readable amount. If there is a lot of microalgae in your aquarium, but your nutrient levels are reading zero, this means the microalgae is out competing your macroalgae for nutrients. Later, we'll talk about how to stop this from happening.
Another common cause is light. It is unlikely that your macroalgae is receiving too much light, depending on the kinds of macroalgae and the schedule you run your refugium/macroalgae scrubber at. Usually, macroalgae dies from not having enough light. This is why you need a high quality refugium light. How do you know if this is the case? Well, if you can eliminate lack of nutrients as a factor, then not enough light is mostly likely the problem. If you have a high quality refugium light such as a Kessil light, there might be something else going on.
The final possible cause is a build of detritus or microalgae on the macroalgae. This will inhibit the macroalgae from getting proper light and growing. It is easy to tell if this is an issue. The macroalgae will appear dirty and full of gunk. This is usually caused by low flow. If you can't seem to stop detritus buildup, you can use a turkey baster to blow off the macroalgae every so often. You can also rinse the macroalgae in a bowl when you harvest it.
How to fight back against microalgae
If your macroalgae is not growing, it is likely being out competed by microalgae. If macroalgae is going to be out competed by microalgae, what's the purpose of using macroalgae? Well, macroalgae is more of a preventative than a solution. Macroalgae will help fight back microalgae but not alone. There are some things you can do to ensure that your macroalgae has the upper hand.
First, you need to make sure your refugium or macroalgae scrubber is a more ideal environment for algae than your display aquarium. You can do this by providing strong lighting and running the lights longer than your aquarium lights. If the macroalgae is receiving less ideal conditions than the microalgae in the display, it only makes sense that the microalgae would out compete the macroalgae.
Another thing that you need to do is physically remove the algae from the main display. You can do this using a brush and glass scrapper. Also, add algae eating animals such as snails, hermit crabs, and tangs. The snails will eat the algae and then produce waste. This may seem counterproductive, but the nutrients from the waste can be consumed by the macroalgae. If you let the microalgae run wild in your tank, it will out compete the macroalgae. However, if you physically remove the microalgae from the tank, the macroalgae will have the upper hand.
Again, macroalgae works best if it is implemented before microalgae takes hold of your display.
Sourcing quality macroalgae
If you have even a small group of reefing friends, chances are that one of them has some macroalgae. They may even offer you some. It is great that hobbyists help each other out, but you should try to get the cleanest macroalgae you can get. This will prevent other alga from entering your tank. It will also ensure that your macroalgae will not be covered with algae that causes the macroalgae to grow slow or not at all.
Another reason it is important to get clean macroalgae is pests. There are a lot of pests that can make their way into your aquarium via your friend's macroalgae.
A great place to get quality macroalgae is the AlgaeBarn itself. AlgaeBarn sells a wide array of different macroalgae in different sizes. You can even find ornamental species of macroalgae such as Dragon's Tongue and Blue Hypnea. Macroalgae from AlgaeBarn is kept in closed systems. They are cleaned, treated, quarantined, and inspected before being shipped out. This level of cleanliness ensures that your macroalgae will have the best chance at growing successfully in your aquarium.
Now you know why your macroalgae may not be growing, and now you know how to solve that issue. Macroalgae is a great tool and can even be used aesthetically, so it is understandable why hobbyists want it to grow the best it can. With some patience and your new (or previously established) knowledge, you'll be able to get your macroalgae doubling in size in no time!